Scott Aaron
MPIfR & JIVE
We present high dynamic range VLBA images of the BL Lac
objects Mrk 501 (1652+398) and 0814+425. Both sources show
a similar total intensity structure: several sharp bends
separated by straight jet segments, eventually broadening
rapidly and becoming diffuse. Furthermore, the polarization
structure of Mrk 501 shows a highly sheared layer at the
edges, which is highly polarized with a magnetic field
parallel to the jet direction, and a "spine" with a magnetic
field orthogonal to the jet flow. We interpret the sharp
bends as oblique shocks as the jet is deflected by clouds
in the ambient material.
|
| The periodicity in the lightcurve of OJ287 | Short talk
| Zulema Abraham
| Instituto Astronomico e Geofisico/University of Sao Paulo
|
The light curve, polarization properties and VLBI structure
of OJ287 are explained in terms of a precessing jet model.
All the geometrical parameters of the jet are determined from
the velocities and position angles of the superluminal
features. The Lorentz factor of the relativistic jet is
completely determined by the shape and duration of the
periodic optical outbursts.
|
| The parsec-scale radio jet of 4C39.25: witnessing the interaction
between a superluminal and stationary component | Poster 1
| Alberdi (IAA), Gomez (IAA), Marcaide (Valencia), Perez-Torres
(Valencia), Marscher (BU)
|
We present multi-epoch 15, 22 and 43 GHz VLBA polarimetric radio observations
of the quasar 4C39.25. With these observations we have been able to:
- Study the geometry of the parsec scale jet of 4C39.25 through the
monitoring of the evolution of the total flux density, the polarized flux
density and -above all- the polarization angle of the region associated
with components a and b; this is of special relevance now that
we have found evidence in favour of an immediate, or perhaps ongoing,
``collision" between the superluminal component b and the region
associated with the stationary component a: i) at millimeter
and centimeter wavelengths, the total flux density light curve has reached a
``plateau" at its maximum value while the polarized flux density is already
decreasing, and ii) the proper motion of 4C39.25 with respect to the
quasi-inertial reference frame defined by the sources observed in the
USNO-programme shows a sharp change in the slope.
- Compare the orientation of the magnetic field between the bent
portions of the jet (associated to components a and c) and
the superluminal component (component b), allowing us to determine
the nature of the stationary components as standing shocks produced
by curvatures in the jet or due to interactions with the surrounding
medium.
|
| Radio Polarization and Flux Density Variability of BL Lacertae:
Oblique Shocks | Short talk
| H.D. Aller, P.A. Hughes, I. Freedman, and M.F. Aller
| University of Michigan
|
The results of more than 20 years of UMRAO multifrequency single antenna
observations are presented which reveal a series of highly polarized outbursts.
Comparison with VLBI maps shows that for many bursts the plane of the electric
vector of the polarized emission is parallel to the observed jet structure, and
several of these bursts have been quantitatively modeled by weak, transverse
shocks propagating along a relativistic flow. However, in the past decade the
polarization position angle has exhibited variations of tens of degrees from
burst to burst during time periods when VLBI observations by Mutel and others
have shown curvature in the radio jet structures. During periods when VLBI
revealed new components, whose direction of propagation deviated by ten or more
degrees from the average direction, the polarization position angles deviated in
the same direction by even larger values. This behavior suggests the presence of
oblique shocks. The degree and direction of polarization together with our total
flux data are used to constrain the possible strength, obliquity and orientation
of the shock fronts. This work was supported in part by NSF Grant AST950178.
|
| Radio-Waveband Flux and Polarization Variability | Invited Talk
| Margo F. Aller
| University of Michigan
|
Centimeter-wavelength flux and linear polarization observations have quantified
the high levels of activity in BL Lacertae objects and identified variability
timescales ranging from < 1 day to several years. These data are consistent
with the frequent occurrence of weak internal shocks in BL Lac jet flows; such
shocks may result from the development of Kelvin-Helmholtz instabilities
(possibly due to only mildly-relativistic bulk motion), and by reflections from
the jet/ambient medium interface. The observed spectral flattening into the
millimeter band, apparent from multifrequency monitoring measurements, is
consistent with a flatter-than-canonical electron energy distribution of the
radiating particles and suggests a class-dependent difference in the underlying
flow compared to QSOs. Spatial variations in opacity as well as jet curvature,
identified by VLBI/P measurements, play significant roles in source emission
and must be accounted for in properly interpreting variability data. Long-term
temporal changes in the flow in some class members, found from analyses of
tri-decade-long monitoring measurements, are only now beginning to be explored.
| Host Galaxies of Flat Radio Spectrum Sources | Short Talk
| Sonia Anton
| NRAL - Jodrell Bank
|
We discuss the host galaxy morphologies of an optically-bright flat spectrum
radio source sample.
Though from their radio properties the sources are essentially identical, all having
structures dominated by flat-spectrum compact radio-cores, optically they are divers
e. In fact from the optical polarimetric and spectroscopic data the sources fall into 4 b
road categories: BL Lacs and BL Lacs candidates, weak emission line galaxies,
sources with Seyfert-like spectra, and hybrid-type objects, ie, with weaker emission
lines that the Sy-type objects but nonetheless broad.
We compare the host galaxy morphology of BL Lac objects with those galaxies host
ing Seyfert-like activity, and with those galaxies which appear to have no
optically active nucleus.
|
| 0.75m robotic telescope for quasar monitoring | Poster 15
| M.K.Babadzhanyants (1,2), M.V.Chernyshov (1,2)
| (1) Astronomical Institute of St.-Petersburg University,
(2) MB Telescope Co.
|
We present the project that aims at renewal and advance the Petersburg
Quasar Monitoring Program (PQMP) having been carried out during 1968-1993
(photographic photometry of blazars, OVV quasars and AGNs using 0.45m telescope at By
urakan Station located in Armenia).
The homogenious data sets for seven basic PQMP objects are of 18-26 years in dur
ation.
The new 0.75m full automated Ritchey-Chretien telescope with remote control had
been designed and is now under construction. We describe some features of telesc
ope design that takes into account the specifics of monitoring observations.
This telescope will be equipped by CCD photometer for multiband (BVRI) monitoring of blazars.
|
| AGNs as a test of relativistic gravity theory | Short Talk
| Yurij Baryshev
| Astronomical Institute of St.-Petersburg University
|
A review of possible tests of relativistic gravitation theory in
the case of strong gravity of the AGN 's central energy source is presented.
These tests are devided into three groups: (1) the testing of equations of
probing particles motion; ( 2) the testing of field equations; ( 3) the testin
g of central compact supermassive object structure.
Geometrical gravity theory (General Relativity) and Field-Theoretical descr
iption of gravitational interaction are compared from the point of thier predict
ions of effects in the strong gravity fields.
Evolution of the binary supermassive systems (such as OJ287), detection of g
ravitational radiation by modern gravitational antennas and disk-surface effects
of the central AGN's engine are discussed.
|
| Evolutionary behaviour of X-ray selected BL Lac objects | Long Talk
| Volker Beckmann and Norbert Bade
| Hamburger Sternwarte
|
We present a new well defined sample of BL Lac objects selected from
the ROSAT All-Sky Survey. The sample is based on an area of about
4000deg-2 at the northern hemisphere. X-ray spectral properties
were determined for each object individually with the RASS data. Our
analysis confirms the negative evolution for X-ray
selected BL Lac objects found in a sample by the Einstein observatory,
the parameterization provides similar results. A subdivision of the
sample into halves according to the X-ray to optical flux ratio
yielded unexpected results. The extremely X-ray dominated objects
have higher redshifts and X-ray luminosities and only this subgroup
shows clear signs of strong negative evolution. The evolutionary
behaviour of objects with an intermediate spectral energy distribution
between X-ray and radio dominated is compatible with no evolution at
all. The distinction between the two subgroups can be explained if
extreme X-ray dominated BL Lac objects are observed in a state of
enhanced X-ray activity.
| Results of 19-years optical monitoring of blazar ON 231.
Analysis of variability. | Poster 16
| E.T.Belokon, M.K.Babadzhanyants
| Astronomical Institute of St.-Petersburg University
|
We present the results of B-band photographic photometry of blazar ON
231 obtained during 1972-1990.
The observations were made using 0.45m reflector of Astronomical Institute of
St.-Petersburg University located at Byurakan Station (Armenia).
Our data set consists of 280 brightness estimates obtained during 131 nights that
were arranged nearly evenly on observing time interval.
The addition of our data to ones published up to now increase twofold the number
of available optical brightness estimates of ON 231 in 70-80s.
The analysis of ON 231 optical variability is presented.
|
| Radio monitoring of Blazars observed by SAX | Poster 11
| 1. H. Bignall 2. A. Tzioumis 3. F. Mantovani, T. Venturi,
D. Dallacasa 4. P. Padovani
| 1. Uni. Adelaide 2. Australia Telescope 3. Istituto di Radio
Astronomia, CNR, Italy 4. STScI
|
A sample of about 55 Blazars is currently under observation
with the SAX satellite. Most of these sources are being
monitored as part of a multiwavelength campaign from X-Ray to
radio wavelengths. The sources in the northern hemisphere
have been monitored in the radio by Medicina for the past
2 years at 5 & 3 cm. The southern sources have been monitored
over the last year using the Australia Telescope Compact
Array (ATCA) at 21, 13, 6 & 3 cm. The ATCA provides both flux
density and polarization measurements.
We will present preliminary results from this radio monitoring
programme.
|
| The spectra of gamma ray blazars. | Poster 31
| C.-I. Björnsson & Torsten Aslaksen
| Stockholm Observatory
|
The spectra of blazars, in particular their high frequency component, are
discussed in terms of multiple Compton scattering models. When trying to
account for the observed spectra, such models have several attractive
features as compared to those including only a single Compton scattering.
The inferred properties of both the electron distribution and magnetic
field (for SSC-models) differ in several important aspects from those
derived under the assumption of a single Compton scattering. Numerical
solutions are shown for an SSC-model where the electrons are injected at a
shock front. An analytical approximation is derived which reproduces well
the numerical results.
|
| Low Luminosity BL Lac objects and flat spectrum radio sources | Poster 2
| M. Bondi
| Istituto di Radioastronomia, Bologna, Italy
|
We obtained two-epoch VLBA observations at 5 GHz of a list
of radio galaxies drawn from the 200 mJy sample (Marcha et al.
1996). The objects selected for milli-arcsecond scale observations
are classified, on the basis of their optical spectroscopic
and polarimetric properties, as BL Lac objects, normal weak
line radio galaxies, broad line radio galaxies, and transition
objects (those with intermediate properties).
We present preliminary results on the radio polarization
properties, on the milli-arcsecond scale, of objects with
different optical properties and discuss structural variations
detected from the two epochs.
|
| ROSAT observations of PKS 2155-304 | Short Talk
| W. Brinkmann, J. Siebert
| MPE Garching, Germany
|
The BL Lac object PKS 2155-304 has been monitored extensively
in May 1996 and November 1996 during a multi-wavelength campaign with
the ROSAT HRI. The monitoring was very 'dense', mostly a pointing for
every satellite orbit.
Thus, we could study the intensity variations of the BL Lac on all
time scales up to days.
The light-curves can be interpreted as the superposition of
individual, single 'shots' occuring on time scales of less than
one day and long term smooth intensity variations.
The short-term variability with time scales of several minutes
cannot be uniquely
attributed to source intrinsic intensity changes as systematic effects
related to the orbital period of the satellite seem to be present as well.
A comparison with the only other 'dense' ROSAT observation with the
PSPC in November
1991 indicates a long term persistence of the character of these daily
variations.
Over all, the soft X-ray flux from the source, observed by ROSAT
repeatedly since 1990, varies by a
factor of two with respect to the average over the years.
|
| The CJF survey - first results on superluminal motion | Short Talk
| S. Britzen, R.C. Vermeulen G.B., Taylor, C.S. Readhead,
T.J. Pearson, D.R. Henstock, P.N. Wilkinson
| NFRA, Dwingeloo; NRAO, Socorro; CIT, Pasadena; University of
Manchester, Jodrell Bank
|
The Caltech-Jodrell Bank flat-spectrum (CJF) sample is a complete
flux-density-limited sample of 293 flat-spectrum sources.
It integrates existing
VLBI survey observations (Pearson & Readhead 1988; Polatidis et al. 1995;
Taylor et al. 1994) into a large, homogeneous
database for statistical studies of a broad range of astrophysical and
cosmological issues; e.g. the combination of the apparent
superluminal motion distribution in the parsec-scale jets with radio
source counts may allow us
to make an assessment of the presence of cosmological evolution in the velocity
distribution.
Most recently third epoch VLBA observations at 5 GHz have been finished.
Here we present first results on the superluminal motion statistics
in view of different identification classes.
|
| New BL Lacs from the REX survey | Poster 39
| A. Caccianiga, R. Della Ceca, I. M. Gioia, T. Maccacaro, A. Wolter
| Osservatorio Astronomico di Brera, Milano; IRA - CNR Bologna
|
We present the preliminary properties of the BL Lacs
discovered in the REX survey (Caccianiga et al. 1998). In
particular, we discuss a few sources with optical spectral
properties ``intermediate'' between those of BL Lacs and
those of elliptical galaxies.
These objects could harbour weak (in the optical band)
sources of non-thermal continuum in their nuclei and, if
confirmed, they could represent the faint tail of the
BL Lac population. The existence of such ``weak'' BL Lacs is
matter of discussion in recent literature (e.g. Marcha et
al. 1996) and could lead to a revision of the defining
criteria of a BL Lac and, consequently, of their cosmological
and statistical properties.
|
| Markarian 421: EUVE observations from 1994 to 1997 | Poster 40
| Cagnoni, I. (1), Fruscione, A. (2) and Papadakis, I. (3)
| (1) SISSA, Italy (2) CfA, USA (3) Univ. of Crete
|
We present spectral and timing analysis of all the data
collected by the Extreme Ultraviolet Explorer for the BL Lac
object Mrk 421 from 1994 to 1997. During these years Mrk 421
has been observed by EUVE 4 times with the DS/Spectrograph
and 2 times with the imaging telescopes for a total of ~1.4
millions seconds. The total EUVE light curve seems to be
smoothly varying on the long time-scale while on a shorter
time-scale there is evidence of an EUVE flare correlated to
the 1995 TeV flare. We analysed the three spectral data set
in an homogenous way using the proper off-axis calibrations
and checked for the existence of absorption features around
70 Å (Kartje et al.,97,ApJ,474,630).
We also show the first power spectrum analysis of the Mrk 421
EUVE lightcurves and a comparison with the predictions by
theoretical models.(e.g. Celotti et al.,1991,ApJ,377,403).)
|
| Optical-UV Observations of 3C 66A | Poster 17
| M. T. Carini, J.C. Noble and H.R. Miller
| Western Kentucky University and Georgia State University
|
We present observations of the variability of the Blazar
3C 66A at both optical and UV wavelengths. Long term optical
photometric observations spanning over 20 years are presented,
along with optical observations of the microvariability
observed in this source. In addition, simultaneous optical and UV
observations of microvariability obtained during a source outburst
will be presented.
|
| Multiwavelength Observations of TeV Emitting BL Lac
Objects | Long talk
| Michael Catanese, for the Whipple Collaboration
| Iowa State University
|
We will present contemporaneous multiwavelength observations
of the TeV sources Markarian 421 and Markarian 501 with
particular emphasis on observations taken in 1995 on
Mrk 421 and in 1997 on Mrk 501. Both show strong correlations
between the TeV gamma rays and lower energy photons, particularly
in the soft to hard X-ray regime on time scales from days
to hours. The correlations between the gamma rays and
lower wavelengths and the extent of the emission from these
objects to >10 TeV imply high Doppler factors and
stringent limits on magnetic fields in the gamma ray
production region. These results and their implications
for BL Lac emission models will be discussed in the
presentation.
|
| Testing the FR I/BL Lac unifying model with HST
observations | Poster 49
| Annalisa Celotti and Alessandro Capetti
| S.I.S.S.A., via Beirut 2-4, 34014 Trieste, Italy
|
HST observations provide a novel way of testing unified models for FR I
and BL Lacs through the direct comparison of any anisotropic optical
nuclear emission present in both systems. Furthermore they revealed the
presence of extended nuclear disks in FR I which might provide information
on the jet orientation.
We selected 5 FR I which show nuclear discs. At their center an
unresolved nuclear source is present. We compared its luminosity with the
optical core luminosity of BL Lacs selected for having similar host
galaxy magnitude and extended radio luminosity. The BL Lac/FR I core
luminosity ratio ranges between 200 and 300000 and shows a clear trend
with the orientation of the radio galaxies with respect to the line of
sight. This behavior is quantitatively consistent with a scenario in
which the emission in the FR I and BL Lac is dominated by radiation
beamed with Doppler factors 5 - 10, thus supporting the proposed
unification scheme.
|
| NEAR INFRARED PHOTOMETRY OF BLAZARS | Poster 18
| Chapuis, C. et al.
| SAp/DAPNIA CEA Saclay
|
One of the most important results of the Compton Gamma Ray
Observatory has been the discovery that blazars (BL Lac
objects and highly polarized quasars) and flat radio
spectrum quasars are strong gamma-ray emitters. Furthermore,
this emission is strongly variable on time scales as short
as days, indicating that the emission region is extremely
compact. It has been shown that the near infrared luminosity
correlates well and better with gamma-ray luminosity than
with X-ray. Two days of observations were conducted at the
Palomar observatory with the Hale telescope, in order to
search for rapid variability in the near infrared bands J,
H and Ks. We deduce the gamma-ray fluxes,
the total luminosity and will discuss our photometry results.
|
| BL Lac objects with distorted radio structure | Long talk
| P. Charlot and H. Sol
| Observatoire de Paris-Meudon
|
Misalignment between VLBI and large scale morphology is
quite frequent in BL Lac objects. EVN data for a sample of
such misaligned BL Lac allow to investigate the intermediate
scale between pc and kpc jets, which can help to discriminate
between different models for the distorsion. This will be
presented and discussed.
|
| TeV emission of Mrk 421 | Poster 41
| K.S. Cheng, W.Y. Mak
| University of Hong Kong
|
Mrk 421 is the nearest Bl Lac object observed by EGRET. TeV
gamma-ray has been observed by Punch et. al in 1992. Montigny
et al. have launched multiwavelength observation in 1994. Only
TeV and keV flare were observed. This favors homogeneous SSC
model. In 1996, Gaidos et. al reported that the time
variability of its TeV can be in order of an hour. This
implies that the emission region is extremely small, probably
smaller than 1016cm. We present a model in which
relativistic mono-energetic electrons are accelerated by
electromagnetic mechanism and they produce highly energetic
photons through synchrotron radiation. These energetic
photons form pairs with soft photons coming from the disk
and pairs produce synchrotron radiation which are inversely
Compton scattered by relativistic electrons along the jet.
Fitting for the spectrum is also presented.
|
| Numerical simulation of blazars rapid variability | Poster 32
| Marco Chiaberge, Gabriele Ghisellini
| SISSA, Osservatorio Astronomico di Brera-Merate
|
Blazars are characterized by large amplitude and fast variability, indicating
that the electron distribution is rapidly changing, often on time
scales shorter
than the light crossing time. We study the time dependent behaviour of the
electron distribution after episodic electron injection phases, and
calculate the observed synchrotron and self Compton radiation spectra. Since
photons produced
in different part of the source have different travel times, the observed spectrum
is produced by the electron distribution at different stages of evolution.
Even a homogeneous source then resembles an inhomogeneous one.
Time delays between
the light curves of fluxes at different frequencies are possible, as
illustrated for the specific case of the BL Lac object Mkn 421.
|
| BATSE observations of BL Lac objects | Short talk
| V. Connaughton, M. McCollough, C.R. Robinson, S. Laurent-Muehleisen
| NASA - Marshall Space Flight Center
|
The Burst And Transient Source Experiment (BATSE) on the
Compton Gamma-Ray Observatory has been shown to be sensitive
to non-transient hard X-ray sources in our galaxy,
down to flux levels of 100 mCrab for daily measurements,
3 mCrab for integrations over several years. We use the
continuous 7-year BATSE database and the Earth Occultation
technique to extract average flux values between 20 and
200 keV from complete X-ray and radio selected BL Lac samples,
and to measure their variability over periods of
months to years. We compare the behaviour and the
spectral characteristics of the objects in the
XBL and RBL classes.
|
| The radio-optical spectra of BLLacs and possible relatives | Short talk
| J. Dennett-Thorpe
| University of Lisbon
|
I consider the suggestion that, in a complete sample of flat-spectrum
radio sources with available optical spectra (Marcha et al 1996), the
strong emission line objects, or those with passive elliptical spectra
are close relatives of the BLLacs.
New observations at four frequencies from 8 to 43GHz are presented, together
with evidence for radio variability. Combined with other radio and
optical data from the literature, we are able to construct the
non-thermal SEDs and use these to address the questions: are the
optically passive objects potentially `unrecognised' BLLacs (either
intrinsically weak and/or hidden by starlight)? What is the
relationship between the surprising number of strong emission-line
objects and the BLLacs?
|
| Comparative Study of Microvariations in Radio Quiet
and Radio Loud Quasars | Short talk
| J.A. de Diego, D. Dultzin-Hacyan, A. Ramirez, E. Benitez
| Instituto de Astronomia - UNAM
|
We have performed photometric optical observations of a
sample of 16 radio loud and 16 radio quiet quasars. The
aims were to confirm the existence of microvariations in
strictly radio quiet objects, and to compare the properties
of the variations in both types of quasars.
The samples were defined in such a way, that the only
observational difference between the two groups is the
radio power.
We present a new observational and statistical procedure for
searching for microvariability, based on the Annalysis of
Variance. We have found that microvariations in radio quiet
objects may be as frequent as in radio loud sources.
|
| Broad Band CCD Photometry and Spectrophotometry of
OJ287 During the Minimum of 1998 | Short Tal
| D. Dultzin-Hacyan(1), Erika Benitez(1),
Jose Antonio de Diego(1) J. Y. Wei (2) & Q. S. Gu(3)
| (1) Instituto de Astronomia, UNAM, Mexico
| (2) Beijing Astronomical Observatory, Beijing, P. R. China
| (3) Dept. of Astronomy, Nanjing University, P. R. China
|
Broad band B V R I, CCD photometry and spectrophotometry observations
are presented for the blazar OJ 287. A nightly monitoring was carried
out at San Pedro Martir and Beijing Observatories for five nights
during minimum brightness of OJ287 (January 1998). The observations are
nearly simultaneous (actualy sequential due to day/night difference)
and the spectral range covers approximately the same range as the broad
band photometry. We report both continuum and line variability, and
discuss our results.
|
| Polarization angle rotation of OJ 287 based on all
available
optical polarimetry | Short talk
| Efimov, Yuri
| Crimean Astrophysical Observatory
|
The rotation of position angle of polarization of OJ 287 was observed
practically during whole time of its first polarimetric observations
covering about 25 years. These observations provided a strong support
to the helical structure of magnetic field in jet. An attempt is made
to study the duration of this continuous rotation in various time
intervals.
|
| BL Lac Objects and relativistic beaming model | Short talk
| J.H. Fan
| CRAL Observatoire de Lyon
|
In this paper
1. The assumption of the dependence of Doppler factor on the
emission frequence
( dv = d01+(1/8)log(gv/4),
Fan et al. 1993 ApJ, 415, 113) has been used to explain the observations difference
between the radio-selected BL lac objects (RBLs) and the X-ray-selected BL Lac
objects (XBLs); the different Hubble diagram; different multiwaveleght correlation;
different regions in the effective spectral index diagram (aRO - aOX);
and the different polarization, whic suggest that the RBls and XBLs are the same.
2. From the analysis of the relation between the infrared magnitude and redshift, it is
proposed that the parent population of BL Lac objects should be FRI radio galaxies
and FRII(G) radio galaxies with the optical spectra of a galaxy.
3. From the superluminal motion, the assumption
( dv = d01+(1/8)log(gv/4) ) has been confirmed.
4. Based on the relation between polarization and the Doppler factor (Fan et al.
1997, A&A 327, 947), it is proposed that the difference between the radio selected
BL Lac objects and OVVs/HPQs is from the fact that OVVs/HPQs have smaller f (
f is the ratio of the beamed luminosity to the unbeamed luminosity in the source frame)
than BL Lac objects, they should be teh same class originally.
|
| Long-term variation of AGNs | Poster 19
| J.H. Fan, G.Z. Xie, G. Adam, Y. Copin, R.G. Lin, J.M. Bai,
and Y.P. Quin
| CRAL Observatoire de Lyon
|
In this paper we will present the long-term variation in the optical and the infrared bands
for some selected AGNs.
1. Some new optical data observed by us have been presented for BL Lacertae (1995-1996)
and OJ 287 (1994-1995), and new infrared data are presented for OJ 287 (Nov. 1995),
which corresponds to the second optical peak (Sillanpää et al. 1996; Takalo et al.
1996) and during last outburst.
2. For objects with long term observations, the Jurkevich's method has been
used to analyses the long-term variation period. It is interesting that the reported
periods of AGNs are of the similar value of about 10 years: 3C 345 11.4 years (Webb
et al. 1988), 3C 120 15 years (Belokon et al. 1987; Hagen-Thorn et al. 1997), ON 231
13.6 years (Liu et al. 1995), OJ 287 12 years (Sillanpää et al. 1988; Kidger et al.
1992), PKS 0735+178 14 years (Fan et al. 1997), NGC 4151 15 years (Fan et al. 1998a),
BL Lacertae 14.0 years (Fan et al. 1998b). Is the mechanism for the long-term variation
the same for different AGNs?
3. The DCF method has been adopted to analysis the variation correlation in the
optical and infrared bands for BL Lac object OJ 287, the results show that these two
bands are strongly correlated, which suggest that the emission mechanism in the two bands
is the same.
4. For the optical and infrared bands, the maximum variations are correlated.
|
| The HST snapshot imaging of BL Lacs. | Long talk
| R. Falomo, JP.Pesce, R. Scarpa, A. Treves and C.M. Urry.
| Observatory of Padova
|
Using Hubble Space Telescope and WFPC2 we obtained short exposure
images for a hundred of BL Lac objects. The targets were centered
mostly in the PC using the F702W filter. Sources from various
samples, including objects of very different redshift (0.05 to 1.2),
have been observed.
The high resolution and homogeneity of the images allow us to address
the properties of the immediate environments of BL Lacs with
unprecedented capability.
We report here the results on the host galaxies and compare their
properties with previous ground based studies.
A number of peculiar features detected, that need follow up investigation,
are also briefly outlined.
|
| Unifying all 'flavours' of Blazars | Short talk
| G. Fossati(1), A.Celotti(1), G. Ghisellini(2), L. Maraschi(2)
| (1)Int'l School for Advanced Studies (2) Brera Observatory
|
We propose a new physical model for the unification of all
Blazars, moving from a systematic study of their broad band
spectral properties. The continuity of properties among
different classes of sources and the systematic trends of
the SEDs as a function of luminosity favor a unified view
of the blazar phenomenon: a single parameter, related to
luminosity, seems to govern the physical properties and
radiation mechanisms in the relativistic jets present in BL
Lac objects as well as in FSRQ. The general implications
of this unified scheme are discussed, in the light also of
a detailed theoretical analysis, based on fitting continuum
models to the individual spectra of most gamma-ray blazars
|
| Rapid Polarization Variability in 0716+714 Far from
the VLBI Core | Long Talk
| D. C. Gabuzda, P. Yu. Kochenov, & T. V. Cawthorne
| Astro Space Center
|
We discuss rapid polarization variability during our 6-cm
global VLBI observations of the BL Lac object 0716+714.
Integrated measurements made with the VLA during the VLBI
experiment indicated that the polarization position angle for
the VLA core rotated through 50 degrees in 12 hours. Our
analysis revealed that this variability occured roughly
30 mas from the VLBI core. This is an extremely unexpected
result, since it is usually believed that intraday variability
can occur only in very compact structures very close to the
nucleus. These variations may be associated with a compact
shock in the jet at a substantial distance from the core.
Independent of the specific nature of the varying feature, it
is clear that compact structures on a range of scales must be
taken into consideration in studies of rapid variability in AGN.
|
| A Viewing Angle - Kinetic Luminosity Unification Scheme For
BL Lacertae Objects. | Long talk
| Markos Georganopoulos & Alan P. Marscher
| Boston University
|
We propose a unification scheme for BL Lac objects (BLs) based on the angle
(Theta) that describes the orientation of the relativistic jet and on the
kinetic luminosity Lkin of the jet. We assume that Lkin
scales with the size of the jet r in a self-similar fashion (Lkin
-> r2 ), as supported by observational data.
The jets are self-similar in geometry and have the same pressure and median
magnetic field at the inlet, independent of size. The self-similarity is
broken for the highest energy electrons, which radiate mainly at high
frequencies, since for large sources they suffer more severe radiative energy
losses over a given fraction of the jet length. The negative apparent
evolution of X-ray selected BLs is explained as a result of positive
evolution of the jet kinetic luminosity Lkin. We review
observational arguments in favor of the existence of scaled-down accretion
disks and broad emission-line regions in BLs. The proposed unification
scheme can explain the lack of observed broad emission lines
in X-ray selected BLs, as well as the existence of those lines
preferentially in luminous radio-selected BLs. Finally, we review
observational arguments that suggest the extension of this unification
scheme to all blazars.
|
| Emission Models for BL Lac Objects | Invited Talk
| G. Ghisellini
| Osservatorio Astronomico di Brera
|
I will argue that the BL Lac and blazar phenomenology can be
accounted for by a sequence in the source power and intensity of
the diffuse radiation field surrounding the relativistic jet.
An increase of the observed power corresponds to a decrease in
the frequencies of the synchrotron and inverse Compton peaks and
to an increase in the ratio of the powers of the high and low
energy spectral components.
Objects along this sequence would be observationally classified
respectively as high frequency BL Lac objects, low frequency BL Lac
objects, highly polarized quasars and lowly polarized quasars.
These information help also to derive the bulk kinetic power of
the jets, in the zone responsible for most of the emission.
It is found that in powerful blazars the intrinsic radiated luminosity
is a very small fraction of the kinetic one, while in less powerful
BL Lac objects the efficiency is much larger.
|
| VSOP and VLBI observations of Mrk421 and Mrk501 | Short Talk
| G. Giovannini et al.
| Istituto di Radioastronomia
|
We observed Mrk501 with VLBI Space Observations (VSOP) at 18
cm on August 97 and will observe it again on April 98. Mrk421
was observed at 6 and 18cm with ground only observations (VLBA
and EVN + MERLIN).
I will present these new observational results to discuss the
parsec scale structure of these 2 nearby BL-Lac type objects.
A comparison with similar results obtained for low power radio
galaxies (FR I) will be made at the light of unified scheme
models.
|
| A Close-Up Look at Superluminal Motion: Subparsec Radio
Observations of 3C120 and its Comparison with
Numerical Simulations. | Short Talk
| Gomez, Marscher, Alberdi, Marti, Ibanez, Marchenko
| Instituto de Astrofisica de Andalucia (CSIC)
|
We present multi-epoch 22 and 43 GHz VLBA polarimetric radio observations of
the radio galaxy 3C120, part of an ongoing monthly monitoring program of this
source since November 1997. Thanks to the proximity of this source (z=0.033)
we can observe the evolution of superluminal components -up to ten, in this
prolific source, with velocities between 2 and 5 c- with the finest linear
resolution, 0.07 pc. These observations are in agreement with our previous
numerical simulations of superluminal sources, revealing the importance of the
hydrodynamical processes that govern the shock evolution, and the powerful
tool that this comparison between observations and simulations represent in
the study of the parsec-scale jets in AGNs.
|
| Analysis of colour variability of blazar 3C66 A in 1993-96 | Short Talk
| V.A.Hagen-Thorn
| St-Petersburg State University
|
The analysis of colorimetric behaviour of blazar 3C66A based
on the data of OJ-94 Project was carried out by Cholonievski
method. It is found that within each season the variability
may be explained by the existence of single additional source
of radiation with variable flux but unchanged spectral shape.
The source has power-law spectrum but spectral indexes are
different for different seasons.
|
| Instabilities in jets | Poster 33
| M. Hanasz and H. Sol
| DARC, Observatoire de Paris-Meudon
|
New results on the Kelvin-Helmholtz instability of jets
surroundede by a slow cocoon will be presented. Two different
modes of the instability, one related to the inner jet and the
other to the cocoon, are clearly identified, together with
mode crossing and interaction between the two modes.
The relative importance of this instability compare to other
instabilities of magnetic origin will be discussed in the context
of BL Lac sources.
|
| CGRO observations of BL Lacs | Invited Talk
| R.C. Hartman
| NASA/GSFC
|
Among the roughly 60 blazars detected by EGRET, about a quarter are
classified as BL Lac objects. Although most are of the RBL subspecies,
a small number of XBLs have now been detected. In general, the BL Lacs
appear to have slightly harder spectra than the other blazars. One
BL Lac object, BL Lacertae itself, received exceptionally good optical
coverage during an EGRET target-of-opportunity flare observation in
1997 July. During a 1-day miniflare in the middle of the EGRET
observation, the optical and gamma-ray fluxes appeared to peak very
close in time, with the gamma rays possibly peaking a few hours earlier
than optical flux. In the only other blazar flare with comparable
coverage, that of 3C 279 in 1996 Feb, there is little evidence for
1-day-scale gamma/optical correlation, although both were bright.
Other gamma-ray characteristics of BL Lac objects, and comparisons
with flat-spectrum radio quasars detected by EGRET, will be presented.
|
| Host galaxies and Environments of BL Lac Objects | Invited Talk
| J. Heidt
| Landesternwarte, Heidelberg
|
Since the meeting on BL Lac objects in Como 10 years ago,
a tremendous amount of data on BL Lac host galaxies and their
environments was published. This has dramatically improved our
understanding on the morphology of the host galaxies of BL Lac objects
and their cluster environment, which will be reviewed.
Special consideration will be given to the question, if interactions or
merger processes might be the basic phenomenon for triggering the
violent activity observed in BL Lac objects. The necessity of
future observing programms using (very) large telescopes will be outlined.
|
| Snapshot images of BL Lacs from the NOT
| P. Heinämäki et al. | Poster 50
| Tuorla Observatory
|
We present preliminary results from an imaging study of ramdomly
selected BL Lacs obtained at the NOT. All these images are taken
in subarc second seeing conditions in the R-band. Several of these
BL Lacs show irregular host galaxy morphology, indicating the
existence of closeby companions and possible interactions; examples
will be shown and discussed.
|
| Large Amplitude Flares on AGNs | Short talk
| Rene Hudec (1), Frederick Vrba (2), Chris Luginbuhl (2)
| (1) Astronomical Institute Ondrejov, Czech Republic,
(2) USNO, Flagstaff, USA
|
We list and discuss large (10 mag and more) amplitude flares observed
on some of
the AGNs. The extended optical monitoring of some of the GRB
(Gamma Ray Bursts) positions has revealed the presence of
large amplitude optical flares (Optical Transients) which have
been identified with faint underlying AGNs/QSOs. Recently,
optical flare has been found also for the QSO 4C49.29 located
inside small error box of GRB960720. We discuss related
observational and statistical issues and suggest the
strategy for further investigations of these events.
|
| Extraordinary Activity in the BL Lac Object OJ 287 | Short talk
| P. A. Hughes
| Astronomy Dept., University of Michigan
|
We present the results of a wavelet transform analysis of
data for the BL~Lac object OJ~287 acquired as part of the
UMRAO variability program. We find clear evidence for a
persistent modulation of the total flux and polarization
with period ~ 1.66 years, and for another signal that
dominates activity in the 1980s with period ~ 1.12
years. It appears that the longer time scale periodicity
is associated with an otherwise quiescent jet, and the
shorter time scale activity is associated with the passage
of a shock, or shocks. The periodic behavior in
polarization exhibits excursions in U which correspond to
a direction ~ 45° from the VLBI jet axis. This
behavior suggests a small amplitude, cyclic variation in
the flow direction in that part of the flow that dominates
cm-wavelength emission.
|
| On the magnetic field configuration in the central cores of
BL Lac objects | Short talk
| Nazar R. Ikhsanov and Lev A. Pustil'nik
| Universitaets-Sternwarte Muenchen, Germany and Lordan Valley
Academic College, Israel
|
Observations of CGRO and VHE Gamma rays revealed BL Lac
objects to be intensive emitters in the ultrarelativistic
energy range. Interpretation of this phenomenon requires a
specific configuration of the magnetic field in the cores
of these objects. We show that the required configuration
forms in the case of strongly inhomogeneous accretion onto
a gravitating center. We describe the resulting configuration
in terms of Z-pinch. Energy release in Z-pinch leads to the
formation of relativistic particle beams and plasma outflows.
The maximum energies of particles accelerated in Z-pinch are
comparable with that observed with the VHE Gamma rays
telescopes.
|
| Environments of QSOs at intermediate redshifts | Poster 51
| K. Jäger(*), K.J.Fricke(*), J.Heidt(+)
| (*) Universitäts-Sternwarte Göttingen, Germany, (+)
Landessternwarte Heidelberg, Germany
|
We present a deep imaging survey of fields around QSOs within the redshift
range 0.660 OSOs) and
ii) mainly radio-quiet QSOs. Observations were made at Calar Alto, ESO and
McDonald Observatory. They are part of a comprehensive program to study QSO
environments over a large redshift range to detect QSO host galaxies and hints
for tidal interactions with close companions, to search for galaxy clusters
around QSOs, and to study the dependence of QSO environments on redshift
and intrinsic QSO properties. Within our data we have detected several
close companion or cluster candidates around the QSOs. We report on our
clustering statistics of the environments and stress briefly follow-up
observations with the new large telescopes (e.g.~VLT, HET).
| The Tuorla Optical Quasar Monitoring Program, Results 1995-1997 | Short Talk
| S. J. Katajainen et al.
| Tuorla Observatory, University of Turku
|
The results of the optical monitoring of quasars in Tuorla
Observatory between September 1995 and May 1997 are presented.
Total number of monitored objects were 28. Some of the objects
were targets of the simultaneous multifrequency monitoring
campaings (as RXTE or EGRET) and some other objects were
monitored as a part of the OJ-94 project. During our monitoring
campaign we measured brightest V-band magnitudes ever in objects
ON 231 (W Comae) and 3C 66A. There were also some quite strong
outburst also in several other sources too (like S5 0716+714,
MK 421, CTA 102 ect.). Typical for almost all objects is fast
flickering and 0.2-0.4 mag flares, there are also some evidences
on short time periodicity in few objects, not yet confirmed.
|
| The variability patterns of synchrotron emission and particle
acceleration in blazars | Short talk
| J.G. Kirk (1), F.M. Rieger (2), A. Mastichiadis (3)
| (1) MPI-Kernphysik, Heidelberg, (2) Universitaetssternwarte,
Goettingen, (3) University of Athens
|
The acceleration of electrons at a shock front can produce
characteristic patterns in the variation of spectral index as
a function of flux. Using a simple model of the acceleration
process, we present an analysis of these patterns and show
how they vary when the shock front is embedded in a
relativistically moving jet. As well as the relatively well-
known and frequently observed "soft-lag", which should arise
in any model which includes synchrotron cooling, shock models
show "hard-lag" behaviour, provided the observing frequency
is close to the maximum permitted by the acceleration process.
In addition, the timescales of flux rise and fall depend on
the motion (if any) of the shock in through the jet.
|
| VLBI polarization observations of IDV sources | Poster 3
| P. Y. Kochenov and D. C. Gabuzda
| Astro Space Center
|
VLA and VLBI polarization observations at 3.6 and 2 cm of the
six IDV sources 0917+624, 0954+658, 1150+812, 1642+690, 2007+777,
and 0804+499 are analysed in our work. We try to study rapidly
variations by monitoring the VLBI polarized structure of IDV sources
when they are changing. We have already used this method for other
sources; for two of them, 2155-152 and 0716+714, we have clear
indications of an intrinsic mechanism for the rapid polarization
alterations in these sources. We hope that new multifrequency
observations carried out in June 1997 will also display something
interesting in behaviour of at least some of the six observed
IDV sources.
|
| Numerical simulations of relativistic fluid jets | Short Talk
| Komissarov S.S. Falle S.A.E.G.
| The University of Leeds
|
The results of recent numerical simulations of relativistic
fluid jets are reviewed. These include the time dependent
simulatons of superluminal radio radio sources and kiloparsec
jets.
|
| Near-infrared imaging of the host galaxies of a complete
sample of flat spectrum radio quasars nad BL Lac objects | Long Talk
| J.K. Kotilainen
| Tuorla Observatory, University of Turku, Finland
|
Results from high resolution near-infrared imaging of 20 flat spectrum radio
quasars (FSRQ; median z = 0.65) and 11 BL Lac objects (median z = 0.19) are
presented. The host galaxy is clearly detected in 6 FSRQs and 7 BL Lacs, and
marginally in 6 other FSRQs. The hosts of BL Lacs and FSRQs are luminous
(M(H) = -26 and -27, respectively), 1-2 mag brighter than a typical field
galaxy L*, and 0-1 mag brighter than brightest cluster galaxies. FSRQ
hosts are intermediate between those of low and high z radio-loud quasars
(M(H) = -26 and -29, respectively), in good agreement with unified models.
Optical/near-infrared colour and colour gradient of the BL Lac hosts are
similar to those of normal ellipticals, indicating that the nuclear activity
has little effect on the host properties. The nucleus-to-galaxy luminosity
ratio of BL Lacs is similar to that of low z radio galaxies, but smaller than
that of FSRQs, suggesting a difference in the intrinsic nuclear brightness or
in the Doppler beaming factor between the two types of blazars.
|
| Long-term 1-22 GHz Spectra Monitoring of BL Lacertae Objects
in 1979-1996 | Poster 4
| Yu.A. Kovalev, A.B. Berlin, N.A. Nizhelsky, Y.Y. Kovalev
| Astro Space Center of the Lebedev Physical Institute, Moscow,
Russia; Special Astrophysical Observatory, Russia
|
Results of observations and analysis of the strong
long-term variability of instantaneous spectra at 5-7
frequencies are discussed for several well known BL Lacs. The
variability is observed as a process of consistent natural
deforming an initial quasi-stationary spectrum of an object by
the variable ``elementary spectra" of a set of ``elementary
outbursts" in time. Any real outburst can be divided to the
such set. The spectrum of the each elementary outburst is
evaluated by the identical low. It is important that the
evolution of the elementary spectra is natural, but the sum of
them can be more complicated, if the elementary outbursts have
a quasi-chance distribution in time. Such behavior of spectra
can be explained, in particular, by the Hedgehog jet model of
BL Lacs.
|
| Detection of a Strong Flare in the BL Lacertae Object 0235+164
at 1-230 GHz in 1997-1998 | Poster 5
| Y.Y. Kovalev et al.
| Astro Space Center of the Lebedev Physical Institute, Moscow, Russia
H. Teräsranta, M. Tornikoski Metsähovi Radio Research Station,Finland
E. Valtaoja, Tuorla Observatory, Finland
|
We report the results of 1-230 GHz observations of the BL
Lacertae object 0235+164 during 1997-1998 with RATAN-600 (1-22 GHz),
Metsähovi (22 and 37 GHz) and SEST (90 and 230 GHz). A very strong radio
outburst, or series of superposed flares, started at the beginning of 1997
and is still growing in early 1998. Different physical models for the
flare evolution are discussed.
|
| Observations of 1-22 GHz Instantaneous Spectra for 50
BL Lacertae Objects in 1997-1998 | Poster 6
| Yuri Y. Kovalev
| Astro Space Center of the Lebedev Physical Institute
|
The new results of the instantaneous spectra observations at 31, 13,
7.6, 3.9, 2.7 and 1.4~cm for 50 BL Lacertae objects, carried out from
March, 1997, to April, 1998, at the radio telescope RATAN-600 are
reported. These observations represent the part of our monitoring program
studying the long term spectra variability of compact extragalactic radio
sources (the total list contains now about 550 objects with declinations
of -30° +42°). We discuss and compare spectral properties
and variability for the samples of BL Lacs, HPQs, LPQs, and radio
galaxies. For example, we infer that the distributions of spectral
parameters are nearly the same for BL Lacs and quasars (K-S test) but the
constructed "averaged" spectrum of BL Lacs is flatter than that of quasars
and radio galaxies. We conclude that the same basic physical model can be
applied to the emission of BL Lacs and quasars in the considered
frequency range, with the commonly suggested difference that probably BL
Lacs' jet axes are systematically more aligned with the line of sight.
|
| Intraday Variability in BL Lac objects and Quasars | Short Talk
| A. Kraus, T.P. Krichbaum and A.Witzel
| Max-Planck-Institut für Radioastronomie, Bonn, Germany
|
Intraday Variability in the radio regime is a common phenomenon in blazars
and is found in about 30% of all objects in this class.
While flux density variations of up to 5-10% are typical,
peak-to-peak-variations of 25% or more have been seen in selected
objects.
These rapid variations lead - for the case of an intrinsic
origin - to brightness temperatures
of up to 1018 - 1021K, violating the inverse
Compton limit of 1012K.
New observations with the 100m telescope of the MPIfR have
revealed variations also in the polarized intensity and
polarization angle. While the former ones occur sometimes with higher
amplitudes than the total flux density variations (in some cases
reaching a factor of two), polarization angle variability
can be of the order of a few tens of degrees.
Here we present an overview of IDV-observations carried out
with the 100m telescope and the VLA
both in total flux density and linear polarization.
Models to explain IDV and to avoid the violation of the
inverse Compton limit will be briefly discussed.
|
| Unusual radio variability in the BL Lac object 0235+164 | Poster 12
| A. Kraus1, A. Quirrenbach2, A. Lobanov1,
T.P. Krichbaum1, P. Schneider3, S.J. Wagner4, J. Heidt4,
H. Bock4, A.Witzel1, M. Aller5 and H. Aller5
| 1 Max-Planck-Institut für Radioastronomie, Bonn, Germany
| 2 University of California, San Diego, USA
| 3 Max-Planck-Institut für Astrophysik, Garching, Germany
| 4 Landesternwarte, Heidelberg, Germany
| 5 University of Michigan, Ann Arbor, USA
|
In October 1992 we investigated a number of Blazars at
three radio-frequencies with the VLA and in the optical R-band
to search for short-timescale variations.
In the peculiar BL Lac object 0235+164 such variability was found.
At least one single event can be identified clearly throughout
all three radio frequencies.
However, the properties of this event are very difficult to
explain with the standard models of blazar variability.
This holds for the time sequence of the flux density maxima
at the respective observing frequencies, and the amplitude and
the duration of the outburst as a function of frequency.
Here we describe the lightcurve analysis
and present possible models for the explanation.
|
| Study of the Temporal and Spectral Characteristics of the TeV Gamma-Radiation
from Mkn 501 and Mkn 421 with the HEGRA System of
Imaging Atmospheric Cherenkov Telescopes | Long Talk
| Henric Krawczynski for the HEGRA Collaboration
| Max Planck Institut fuer Kernphysik, Saupfercheckweg 1,D-691
17 Heidelberg, Germany
|
The HEGRA System of 4 Imaging Atmospheric Cherenkov Telescopes with an
energy threshold of about 500 GeV, angular resolution of 0.13 degree
energy resolution of 20% and flux sensitivity of approximately
1.5E-11erg/(cm² sec) (> 500 GeV) for 1 hour observation time has been
used since
January 1997 for a comprehensive study of the TeV gamma-ray emission from
the two BL Lac objects Mkn 501 and Mkn 421. While in 1997 Mkn 501 was in a
very high state with an average flux of about 3 Crab and showed dramatic
flux variations on one day time scale, Mkn 421 was at a rather modest flux
level, one order of magnitude below Mkn 501. Based on a large 1997/1998
data base of approximately 150 hours observation time per source, we
report detailed spectral information for both sources and compare the
temporal and spectral characteristics of these two BL Lac objects. We
briefly discuss the implications of these results on the physics of
relativistic jets in Active Galactic Nuclei.
|
| NEW EVIDENCE FOR THE SHOCKED JET MODEL:
SELF-SIMILAR SEPARATE FLARE COMPONENT | Poster 34
| M. Lainela, A. Lähteenmäki*, H. Teräsranta*, E. Valtaoja
| Tuorla Observatory, *Metsähovi Radio Research Station
|
The shock waves in the relativistic beamed jets are the
standard explanation for radio outbursts in active galactic
nuclei (AGN). We have developed a new simple method which
supports the standard theory. We have decomposed the total
flux lightcurves of over 100 AGNs into a number self-
similar flare components. The flare components consist of
exponential rise and decay parts, where the decay time is
1.3 times the rising time.
The fits are very good in most cases. This supports idea
that one single mechanism is responsible of the radio
outbursts in AGNs.The sources, which are observed frequently
in VLBI monitoring, show good correlation between our
decomposed components and VLBI components.
|
| Near Infrared observations of BL Lac hosts | Short talk
| Georg Lamer
| University of Southampton
|
We have obtained high resolution K-band images of 13 BL Lac
objects and of a sample of FR I radiogalaxies
using UKIRT with the tip-tilt system.
We have been able to resolve the host galaxies of most
of the BL Lac objects and determined their morphologies.
With few exceptions the hosts have been found to be
elliptical galaxies.
We compare the properties of the BL Lac host with those
of the radiogalaxies and discuss the results in the
context of Blazar unification models.
|
| New BL Lac Samples: Searches, Results and Implications
for Unified Schemes | Short talk
| S. A. Laurent-Muehleisen
| Lawrence Livermore National Lab
|
The advent of large area deep radio and X-ray sky surveys is leading to the
creation of many new BL Lac samples. In particular, the Rosat All-Sky (RASS)
and the VLA's FIRST (Faint Images of the Radio Sky at Twenty centimeters)
surveys are proving to be rich sources of new BL Lacs. We will discuss the
methods used in several independent BL Lac searches based on these surveys,
including the RASS-Green Bank, RASS-FIRST, and FIRST Flat Spectrum samples in
addition to the large sample of serendipitous BL Lacs discovered in the course
of the FIRST bright quasar survey. We will present the multiwavelength
properties of these objects, including their broadband radio, optical and
X-ray flux distributions, and their optical emission line and polarization
properties. Comparison of these properties with those of previously known
BL Lacs clearly points to the existence of a large previously unrecognized
population of intermediate BL Lacs: objects with characteristics intermediate
between those exhibited by Low and High energy peaked BL Lacs (LBLs and HBLs,
respectively). We will also discuss what implications these objects have for
the X-ray-leading vs. radio-leading models which attempt to unify the BL Lac
population.
|
| RXTE monitoring of the blazar 3C 279 and 3C 273 | Poster 42
| A.J. Lawson, I.M. McHardy, A.M. Newsam
| University of Southampton
|
In blazar research the key to answering such questions as
`What is the origin and emission mechanism of the high
energy photons?' are the time lags between different
wavebands. Progress towards measuring these lags has been
hampered by the difficulty of obtaining longterm high energy
monitoring, but this situation changed with the launch of
RXTE. We report on and discuss the results we have obtained
from 18 months of monitoring of 3C 279 and 3C 273 by RXTE in
conjunction with other wavebands. Highlights include
simultaneous flares in the X-ray/gamma-ray and X-ray/IR, both
exciting results.
|
| Variability of OJ 287 | Short talk
| Harry J Lehto
| Tuorla Observatory, Piikkiö
|
The characteristics of optical V-band light curve are
investigated and comapred with the light curves from
other optical bands.
|
| Variability models of gamma-ray blazars | Long Talk
| Amir Levinson
| Tel-Aviv University
|
A large fraction of the AGNs detected by EGRET exhibit strong
variability in many wavebands- from radio to the highest
energy gamma-rays observed. The origin of the transient emission
in blazars is presumably associated with relativistic jets.
In this talk I will discuss plausible variability mechanisms that
may operate in blazars. In particular, dissipative fronts produced
by unsteady ejection of magnetized, relativistic outflow, one zone
emission models with time-dependent particle acceleration, and
Compton scattering of external radiation with time varying intensity
will be considered. The implications for the TeV BL Lac objects
will be emphasized.
|
| Spectral Energy Distribution (SED) of EGRET-Detected
Blazars | Poster 35
| Y. C. Lin
| Stanford University
|
According to the current understanding of the electromagnetic
radiation of blazars, the broadband spectrum of a blazar
consists of two general components:(a) a low-energy component
which is believed to be the result of synchrotron radiation
of a beam of relativistic particles and which peaks, in the
spectral energy distribution (SED), in the IR to soft X-ray
region; (b) a high-energy component which is likely the
result of inverse Compton scattering of the same beam of
relativistic particles on some ambient field of soft photons
and which peaks in the GeV/TeV region. The positions of
these two peaks in SED are thus closely related to each other
and are also related to other basic properties of the
radiating blazar. This general picture will be examined
here in the light of EGRET-detected blazars.
|
| Radiospectra and Kinematics in Blazars | Short Talk
| Andrew Lobanov
| Max-Planck-Institut fuer Radioastronomie
|
Radio spectra of total emission from blazars can be used
in combination with kinematic information inferred from
VLBI studies. Such a combination provides a means for
constraining the models suggested for explaining both the
total emission variations and the observed kinematic
properties of compact jets in blazars. In this contribution,
we discuss a method for combining the spectral and kinematic
data, and present results obtained for several prominent
objects.
|
| Comparison of radio and EGRET gamma-ray properties between
BL Lacs and other AGN | Short Talk
| A. Lähteenmäki Metsähovi Radio Observatory
Helsinki University of Technology and E. Valtaoja
Tuorla Observatory University of Turku
|
We have compared EGRET Phase 1+2+3 gamma-ray data with Metsähovi
radio monitoring data of AGN at 22 and 37 GHz. The most probable
gamma-ray AGN to be detected is a high polarization quasar (HPQ)
with an ongoing and rising high frequency radio flare and a large
associated variability brightness temperature T(b,var) (or Doppler
boosting factor D). BL Lacs have a low detection rate compared even
to 'non-blazar', low polarization quasars (LPQs). The detection
probability depends on T(b,var) and, during a single EGRET pointing,
the phase of the radio flare. The strength of the gamma-ray emission
depends also on T(b,var) and on the strength of the radio flare.
The gamma-ray properties of BL Lacs as well as other AGN are closely
related both to the average radio properties and to the individual
radio outbursts (shocks) growing in the source. Natural explanation
is that the gamma emission is produced in the same shocks, induced
by the synchrotron self-Compton mechanism.
|
| X-ray Variability of BL Lacertae in 1997 Outburst | Short talk
| F. Makino
| Institute of Space and Astronautical Science, Japan
|
X-rays from BL Lacertae were observed by X-ray astronomy
satellite ASCA during 18.6-19.6, July, 1997. The X-ray flux
was about four times higher than that observed in 1988 with
Ginga and showed variation of factor 2 during the observation.
The spectra were of power law type which indices changed in
the range of 1.24-1.60. The amplitude of variation in 0.7-1.5
keV was larger than that in 3-7 keV. The spectra can be
expressed by superposition of two independently variable
power law type spectra which could be attributed to
synchrotron and inverse-Compton component respectively.
Multifrequency spectrum will also discussed.
|
| X-ray Variability in BL Lac Objects | Poster 44
| Fumiyoshi Makino
| Institute of Space and Astronautical Science, Japan
|
X-ray spectra of BL Lac objects are classified into two
categories, flat spectrum with index of about 1.7 and steep
spectrum with index of about 2.5. The steep spectrum can be
connected smoothly from radio to X-ray, while the flat
spectrum is discontinuous to the spectrum in UV and optical.
Typical sources of steep spectrum are PKS 2155-304 and Mkn
421. X-ray variation of these sources characterized by
correlation between flux and spectral index and soft lag. The
spectrum becomes steeper with decreasing flux and variation
of flux in lower energy delays from that in higher energy. It
will be shown that these properties are quantitatively
reproduced by synchrotron cooling model by solving time
dependent diffusion equation of electrons for various
injection function. The magnetic field strength can be
derived from the lag.
The solution could be useful for observationa study of
acceleration of the elcrons in BL Lac objects.
|
| On the evolution of BL Lac objects | Poster 52
| D. Malquori, A. Cavaliere
| Universita di Padova, Universita di Roma
|
We study the evolution of BL Lac objects, taking into aacount both
accretion of matter onto the balck hole, and extraction of the
balck hole energy vis the Blanford-Znajek mechanism. We assume,
according to observational evidence, that BL Lacs are viewed in the
beaming axis and that the external conditions provide low mass inflow.
We show that the time scale tL for energy extraction is long
enough to explain why the BL Lac population has very little, if any,
evolution, at strong variance with the rest of the AGNs.
|
| FAST PHOTOMETRY OF THE BL LAC OBJECT
PKS 2155-304 | Poster 20
| Luciano Mantegazza, et al.
| Observatory of Brera, Merate (Como), Italy
|
PKS 2155-304 is the brightest and best monitored BL Lac at the optical and
UV wavelengths. Significant variability in this spectral region on time scales
from years down to a few hours has been commonly observed in this source.
Recent studies have shown that extremely rapid (time scales of ~1 hour or less)
optical and UV flux variability can occur. We have undertaken a program of
systematic intensive optical monitoring of PKS 2155-304 in search of fast
variability events. Intensive CCD photometric monitoring in the V band
accomplished at the ESO 0.91m Dutch telescope in October 1996 during ten days
shows a 20% decrease of the flux in two days, followed by a similar
increase and a subsequent further fading, which determined an oscillating
behavior of the light curve. Intranight variability mostly follows the
daily trend. Results of further V and R observations of PKS 2155-304 taken
in August 1997 will be also presented.
|
| Short-time structural variation of 3C273 | Short talk
| Franco Mantovani et al.
| Istituto di Radioastronomia, Bologna, Italy
|
The results of VLBI observations of the quasar 3C273 made at
22 GHz with a Global Array and at 43 GHz with the Very Long
Baseline Array are presented. The resolution achived with
both arrays is about 0.3 milli-arcsecond.
The source 3C273 has been observed every 10 days for 5 times
at both frequencies during a multi-frequency campaign in late
1992 in the radio, millimeter and X-ray bands.
In order to study the structural evolution of 3C273 the
morphology of the source at each epoch is described using
gaussian components derived by modelfitting the final visi-
bility of each data set. The results will be discussed.
|
| The co-existence of bl lacs and broad lines | Short Talk
| M. Marcha
| univ. of Lisbon
|
Historically, BL Lac objects are flat radio spectrum
sources that do not
show strong emission features in their spectrum. This feature, along
with other radio properties, contributed to the suggestion that BL Lacs
are really the cores of low luminosity radio galaxies (FRI type) but
which are viewed along the direction of the radio jet emission. Such
'unification' of BL Lac objects and FRI galaxies has by now established
itself as the existing scheme for low luminosity radio sources.
However, recent observations have not only shown that the prototype of
BL Lacs has broad emission lines, but also that as we select weaker
radio sources, the separation between objects with broad and narrow emission lin
es becomes less clear. By studying a new sample of low
luminosity, flat radio sources, we investigate whether the host galaxies and clo
se environments are a determinant factor in this behaviour.
|
| Three methods of time series analysis of the blazar's light curves.
Results for OJ 287 and BL Lac in the optical region. | Short Talk
| Marchenko S.G., V.A. Hagen-Thorn, V.A. Yakovleva
| Astronomical Institute S.-Petersburg University
|
The results for search of periodicity by methods of ``Whightening'' and
Wavelet-analysis are represented for the optical light curves of OJ 287 and
BL Lac. The advantages and disadvantages of the methods are discussed.
The comparison of the results obtained on the base of the optical light
curves 1967-1989 (1) and the same ones supplemented by teh data of the
OJ-94 archive are carried out. The possible fractal character of the
blazar's light curves is supposed. In the frame of the fractal analysis
Hurst's parameters and the fractal dimension of the light curves
are estimated.
(1) Marchenko, S.G., et al. 1996, PASP conf. Series, vol.110, eds. H.R. Miller,
J.R. Webb and J.C. Nobble, p. 105.
|
| Near-IR imaging of OJ287 and PKS 1510-089 | Poster 56
| Marchenko S.G.
| Astronomical Institute S.-Petersburg University
|
The near - IR observatioms of two blazars OJ 287 and PKS 1510-089
were carried out on 1997 February 16-18 at 3.8 United Kingdom
Infrafed Telescope.
The composite K-band images show evidence for existence of a host
galaxy both for OJ 287 and PKS 1510-089. The absolute K-magnitude of the
underlying nebulosity for OJ 287 lies in the range from -26.1
to -25.5. An offset in the centroid position between the OJ 287 point
source and the underlying nebulosity is found and measured to be about
0.4 arcsec at northwest direction.
|
| A Monte-Carlo method for particle acceleration at multiple
shocks in blazar jets | Poster 36
| A. Marcowith, J. Kirk
| Max-Planck-Institut fuer Kernphysik
|
We present a new Monte-Carlo method for particle acceleration
and apply it to multiple shocks. These calculations are
relevant to blazars, since they extend the single region
"homogeneous models" to include multiple emission regions.
Previous analytic and numerical work on multiple shocks has
assumed them to be well separated in time or space. We lift
this restriction by using a system of stochastic differential
equations equivalent to the diffusion-convection equation for
energetic particles (Kruells & Achterberg 1994) and
implementing a new semi-implicit integration method. The
results exhibit the flat spectrum implied by blazar radio
emission together with a piling-up effect due to synchrotron
losses. At even higher momenta, single shock acceleration
takes over and the spectrum shows a power-law tail, which may
be relevant to the hard X-ray emission from Blazars.
|
| The parsec-scale jets of BL Lac objects | Invited Talk
| Alan P. Marscher
| Boston University
|
I will discuss the parsec-scale jets of BL Lac objects,
emphasizing what we have learned from high-frequency VLBI observations.
I will compare the behavior of BL Lac objects with that of quasars, based
on a sample of over 40 gamma-ray bright blazars. The very striking alignment
of the polarization position angle with the jet axis seen in BL Lac objects
at 5 GHz disappears at high frequencies. For many BL Lac objects, the jet
appears very similar from one epoch to the next, although closely spaced
monitoring reveals changes.This seems to indicate that we are seeing
disturbances (e.g., shocks) propagating through an underlying jet pattern
that changes little with time.
|
| BVRI photometry Of the gamma-ray loud BL Lac object
S5 0716+714 | Poster 21
| E. Massaro, M. Maesano, F. Montagni, R. Nesci, G. Tosti,
M. Villata, C. Raiteri, G. Sobrito
| Ist. Astronomico, Univ. di Roma - Oss. Astron. Univ. di Perugia -
Oss. Astr. di Torino
|
The optical activity of the bright gamma-ray loud blazar S5 0716+714
is monitored by the Perugia-Roma-Torino collaboration since November
1994. In this contribution we update the light curve presented by
Ghisellini et al. (1997, A&A 327, 61) to the spring of 1998.
The source was often in very bright luminosity states and reached
the highest level ever observed (September 1997).
In a few occasions we carried out simultaneous observations with
a few telescopes to study intranight variability in different bands.
Flux changes with a typical amplitude of about 0.1 mag (R) have been
often detected.
|
| Structure and timescales of the fast variations of BL Lacertae
during the 1997 Summer active phase | Short talk
| E. Massaro(1), M. Maesano (1), F. Montagni (1), R. Nesci (1),
G. Tosti (2), M. Fiorucci (2)
| 1. Ist. astronomico, Univ. La sapienza, Roma - 2. Oss. Astronomico, Univ. di Perugia
|
We present the results of a time analysis of some intranight light
curves of BL Lacertae during the very active phase of June-September
1997. In particular, we studied the luminosity variations over time
scales from 0.3 to 3 hours, after subtraction of longer trends.
the data have been analyzed with several methods, including the
non-linear Prony algorithm, to investigate the relation between the
aplitudes and the durations of the variations. A non periodic
oscillating behaviour with an approximate time scale of about two
hours has been noticed in a few occasions.
Possible implications on physical models based on synchrotron
emission in a strong turbulent magnetic field are discussed.
|
| X-Ray Variability of BL Lacs | Invited Talk
| Ian McHardy
| University of Southampton
|
I present an overview of the X-ray temporal and spectral variability
of BL Lacs on both short and long timescales. The previously observed
behaviour of short (~days) flares superimposed on a relatively steady
`quiescent' level is still broadly correct. However, for the brighter
BL Lacs, the well sampled lightcurves from the RXTE ASM show that the
`quiescent' level also varies considerably on timescales of ~100 days
in a manner similar to that seen in Optically Violently Variable
Quasars (OVVs) such as 3C279 and 3C273. Possible reasons for this behaviour are
discussed. For the large majority of BL Lacs the soft and medium
energy X-ray bands are dominated by synchrotron emission and, unlike
the case of OVVs, the emission mechanism is not in doubt. Most
interest then centres on the structure of the emitting region, and the
electron acceleration processes, particularly during outbursts. That
structure, and the acceleration processes, can be investigated by
consideration of the spectral variability during flares, which is not
simple. I review the observations of spectral variability and consider the
evidence for and against homogeneous models.
I also briefly compare the X-ray spectral variability of BL Lacs with that of
OVVs such as 3C273.
|
| THE CHARACTER OF THE OPTICAL VARIABILITY OF
MRK 421, MRK 501 AND 1ES 2344+514 | Short Talk
| H. Richard Miller, Alyssa B. Daya and Elizabeth C.Ferrara
| Georgia State Univ.
|
The Optical variability of the three TeV blazars, Mrk 421, Mrk 501 and
1ES 2344+514, has been investigated on timescales ranging from minutes
to decades. The character of the variations has been investigated and
compared with that observed for radio-selected BL Lacs. These results
will be discussed, and the implications for the X-ray selected BL Lac
(XBL) / radio-selected BL Lacs (RBL) dichotomy will be discussed.
|
| "THE MICROVARIABILITY OF AO 0235+164 IN OUTBURST
AND QUIESCENCE" | Poster 23
| H. Richard Miller, Georgia State Univ., John C. Noble,
Western Kentucky Univ., and Robert E. Fried, Braeside Observatory
|
The optical microvariability of AO 0235+164 has been studied during
outburst with R ~ 14.0, at a quiescent stage near R = 19.0, and at an
intermediate stage with R = 17.5. The character of the variations has
been investigated and compared for observations made at each of these
levels. The results of these observations and analyses will be
presented, and the implications for the physical conditions and
emission processes present in each state will be discussed.
|
| On the contribution of unresolved blazars to the extragalactic
gamma-ray background | Short Talk
| (1) A.Muecke , (2) M.Pohl
| (1) The University of Adelaide, (2) Danish Space Research
Institute
|
The discovery of over 60 blazars (e.g. BL~Lacs, FSRQs) by EGRET
suggests the idea of a superposition of unresolved AGN as the
origin of the extragalactic gamma-ray background (=EGRB).
To estimate the contribution of unresolved FSRQs and BL~Lac
objects to the EGRB in the EGRET energy range we present a
model which is based on the non-thermal leptonic emission
processes known to be important in blazar jets and the
unification scheme of radio-loud AGN.
According to this picture BL~Lacs and FSRQs are the parent
population of FR~I- and FR~II-radio galaxies, respectively.
Using the well-known poperties of these FR-galaxies we
calculated the gamma-ray emission of these objects which
fall below the EGRET-sensitivity limit due to a large viewing
angle and/or a low intrinsic luminosity.
Recent results are presented.
|
| BL Lac Objects in a complete sample of X-ray Sources | Poster 45
| R. Mujica, F.-J. Zickgraf
| Observatoire Astronomique de Strasburg
|
We have identified and studied the BL Lacs in a subsample of
X-ray sources detected during the ROSAT All-Sky Survey.
We exploited their intrinsic broad-band and variability
properties to reliably identify them. We found that several
of our objects fall in a region that was previously thought
to be empty in the aox -aro diagram that can
be interpreted as a transition between the two classes of
(RBLs and XBLs). We also found correlations between the
spectral indices aox and aro and the amplitude
of the variations.
|
| Optical photometry of the BL Lac object S5 1803+78 in 1996/8 | Poster 22
| R. Nesci, E. Massaro, M. Maesano and F. Montagni
| Instituto Astronomico, Universita' "La Sapienza", Roma ITALY
|
We present the preliminary results of a BVRI photometric monitoring
of the circumpolar BL LAc object S5 1803+78 started in April 1996.
The source luminosity showed an overall variation of about 2.4 mag
in all the bands. The brigthest level (R=13.8) was observed in
September 1997 and after that the flux declined for 20 days at a
mean rate of 0.07 mag/day.
|
| High resolution imaging of TeV BL Lacs | Short talk
| K. Nilsson, T. Pursimo, L. Takalo, A. Sillanpää and J. Heidt
| Tuorla Observatory
|
We have obtained deep subarcsecond optical images of three BL Lacertae
objects, Mkn 421, Mkn 501 and 1ES2344+514, that have been detected at TeV
gamma-ray energies. We discuss the properties of their host galaxies
and immediate surroundings and give structural parameters and photometric
correction tables for each object.
|
| A Comparison of the Optical Microvariability Morphology in
X-ray-selected and Radio-selected Blazars | Short Talk
| John C. Noble
| Western Kentucky University
|
Blazars are widely known to exhibit variability on many different
timescales. The term microvariability is used to describe variations
that occur on the order of seconds to hours. We have compiled an
extensive database relating to the optical microvariability
phenomenon in blazars and have undertaken an effor to characterize
its morphology. Here we use this morphological characterization in
order to investigate possible differences in the microvariability
phenomenon between x-ray-selected and radio-selected blazars.
We do this in the hope of being able to put constraints on the
mechanism responsible for producing such variations so that we can
better understand the nature of these objects.
|
| RECENT OBSERVATIONAL CONSTRAINTS ON THE WARM
ABSORBER IN AGN | Poster 46
| A. Orr
| Astrophysics Divsion, Space Science Department of ESA
|
Among the most prominent observational signatures of the partly ionized component
in active galactic nuclei (AGN), the so-called warm absorber, are spectral
absorption features detected in the keV region.
These features provide important diagnostics on the location and nature of the
material surrounding the active nucleus.
The Low Energy Concentrator Spectrometer (LECS) on board the BeppoSAX
satellite operates in the range 0.1-10 keV. Because of its good low-energy
resolution and effective area down to 0.1 keV
it is a very efficient tool for the study of the complex spectral features
due to warm absorbers, particularly when used in combination with the other
narrow field instruments on BeppoSAX, sensitive at higher energies.
We will present here new constraints on the physical state of the warm absorber
in AGN brought by BeppoSAX/LECS observations as well as by recent results from other X-
ray observatories.
|
| Monitoring the high energy branch of S5 0716+710 | Short Talk
| Otterbein, Kai
| Landessternwarte Heidelberg
|
The emission processes in active galactic nuclei and
particularly of blazars are still far from clear. Their
extraordinary high luminosity and their strong variability on
very short time scales favour an origin of the radiation
within a relativistic jet. Despite the general agreement of
this picture a variety of competing models based on the
relativistic jet scenario are capable to explain the spectral
energy distribution of blazars for a single epoch.
Investigating the spectral evolution of pronounced
variability patterns throughout the spectrum will be a more
sensitive test to the models. We have monitored the strongly
variable BL Lac object S5 0716+710 ranging from the mm-regime
to the gamma-rays. We present the spectral evolution of the
high energy regime up to the EGRET band and discuss the
impact of our results on recent models for blazar emission.
|
| BL Lacs and Unified Schemes | Invited Talk
| Paolo Padovani
| Space Telescope Science Institute, USA
|
In this presentation I will briefly summarize the main tenets
of unified schemes of BL Lacs, and then review some recent
developments in the field, concentrating mostly on the
results of new, deeper BL Lac samples.
|
| BeppoSAX Observations of 1 Jy BL Lacertae Objects | Poster 48
| Paolo Padovani et al.
| Space Telescope Science Institute
|
We present preliminary results of BeppoSAX observations of
seven BL Lac objects selected from the 1 Jy sample. All
sources characterized by a peak in their multifrequency
spectra at infrared/optical energies seem to show a
relatively flat (ax 0.7 - 0.8) X-ray spectrum.
This is in agreement with the supposed dominance of
inverse Compton emission in the X-ray band of these sources.
|
| Constraining BL Lac objects' models using structure
function analysis | Short talk
| Stephane Paltani
| ISDC / Geneva Observatory
|
The structure function (SF) analysis is a very convenient
tool to determine several important properties of a light
curve. We show which parameters can be reliably determined
from the SF, and compare this method with a Fourier analysis.
Applications of SF analysis to light curves of several BL Lac
objects are presented. The presence or absence of shortest
and longest variability time scales are discussed, in
particular in PKS 2155-304, where we show that there is no
variability on time scales comprised between 40 days and 40
years. Always using SF analysis, we can constrain the
Fourier power spectra of the sources. We compare the SFs of
a single object at different wavelengths, and also the SFs of
different objects at the same wavelengths. We show that the
similarities and differences between the SFs have an impact
on the relevance of the models of BL Lac object emission.
|
| Observations of BL Lac objects with the HEGRA
Cherenkov telescopes | Poster 47
| Dirk Petry
| University of Wuppertal
|
Among the now 4(+7) certain sources of gamma radiation above 500 GeV,
2(+1) are BL Lac objects (the numbers in brackets are sources which need
independent confirmation), and these are at the same time the only known
extragalactic sources of TeV Gamma rays. All detections have so far
been made using the ``atmospheric Cherenov technique'', i.e. by
so-called imaging atmospheric Cherenkov telescopes (IACTs). One of the
large experiments using IACTs is the HEGRA installation which is
operating six IACTs as part of its cosmic ray detector complex at the
ORM (Observatorio Roque de los Muchachos) on the Canary island La
Palma. The talk will give a summary of the results obtained so far.
|
| The MAGIC Telescope - bridging the gap at 10 - 200 GeV | Short talk
| Dirk Petry (for the MAGIC Telescope group)
| University of Wuppertal
|
The Major Atmospheric Gamma-ray Imaging Cherenkov Telescope will explore
the energy range above 10 GeV with unprecedented sensitivity. It is thus
going to close the observational gap between 10 and 200 GeV and create
an overlap between ground-based instruments and satellites. The low
threshold will be achieved by the combination of a large mirror (17 m
diameter) and an advanced camera consisting of approx. 450 hybrid
photomultipliers. When observing at large zenith angles, the telescope
will reach extreme collection areas of the order of 1 km² for energies
above 1 TeV. BL Lac objects and Blazars in general will be among the
prime targets for this new instrument. The final decision on the start
of the construction phase is expected before June 1998.
|
| BeppoSAX Observations of the BL Lac Mkn 501 | Short talk
| Elena Pian et al.
| ITESRE-CNR, Bologna, Italy
|
The BL Lac object Mkn 501, which is a source of TeV radiation, was observed
with the BeppoSAX satellite on 7, 11, and 16 April 1997 during a phase of
high activity at TeV energies, as monitored with the Whipple, HEGRA and CAT
Cherenkov telescopes. Over the whole 0.1-200 keV range the spectrum was
exceptionally hard, indicating that the X-ray power output peaked at (or above)
~100 keV. This represents a shift of at least two orders of magnitude with
respect to previous observations of Mkn 501, a behavior never seen before in
this or any other blazar. The correlated variability from soft X-rays to the
TeV band points to models in which the same population of relativistic electrons
produces the X-ray continuum via synchrotron radiation and the TeV emission by
inverse Compton scattering (limited by the Klein-Nishina regime) of the
synchrotron photons or other seed photons. For the first time in any blazar the
synchrotron power is observed to peak at hard X-ray energies. These findings
will be compared with BeppoSAX observations to be taken in spring 1998.
|
| VLBA observations of X-ray selected BL Lac objects from the EMMS survey | Poster 8
| A.G. Polatidis, J.E. Conway
| Onsala Space Observatory
|
We present VLBA observations of a sample of X-ray selected BL Lac objects
selected from the EMSS survey. Due to the weak radio emission of these BL
Lacs (< 40mJy) we made the images using the phase-referencing technique. Most
of the objects appear unresolved, but one objects shows a core-jet radio
structure and a hint for a jet appears in the image of a second object.
|
| Nordic Optical Telescope images of the 1 Jy BL Lac objects | Short talk
| T. Pursimo et al.
| Tuorla Observatory
|
We present results of deep imaging of radio loud BL Lac objects.
Our sample consists of all the objects in the 1 Jy sample
(Stickel et al. 1991, ApJ, 374, 431)
observable from the Canary Islands.
The observations were carried out with the Nordic Optical Telescope
in subarcsecond seeing conditions.
The average integration time exceeds one hour.
The properties of the host galaxies of BL Lac's and
their close environments will be discussed
|
| Polarimetric identification of the RGB BL Lac candidates | Poster 53
| T. Pursimo et al.
| Tuorla Observatory
|
We present preliminary results on our programme "Identification of
new RGB BL LAC objects". The original sample was selected by
crosscorrelating the ROSAT All Sky Survey and Green Bank 5 Ghz
radio survey. In the flux ratio distribution log(Sx/Sr) RGB sample is
intermediate between XBLs and RBLs. Preliminary results suggest
that about 60% of the objects have high (P > 3%)
polarization, confirming their identification as BL Lacs.
The results will be discussed in connecting RBL and XBL subsamples and
compared to previous polarimetric studies of BL Lacs.
|
| VLBI Polarization Observations of a Complete Sample of
BL Lac Objects | Poster 9
| A. B. Pushkarev and D. C. Gabuzda
| Astro Space Center, Moscow
|
Our analysis of first-epoch 6-cm global VLBI observations
for all sources in the Kuhr and Schmidt 1-Jy sample of
northern BL Lac objects has resently been completed. We are
now working on second- epoch global observations, as well as
multi-frequency VLBA observations. We will present our most
recent results from these VLBI studies. In addition, we
present integrated rotation measures (RM) for 15 sources
in the sample (for which values have not been previously
published), based on multi-frequency VLA polarimetric
observations at 18 and 22 cm. These complete integrated RM
measurements for all sources in the sample. The integrated
RMs are probably dominated by the passage of radiation
through the interstellar medium of our Galaxy.
|
| Blasars variability from geodetic VLBI-data | Poster 10
| T.Pyatunina
| Institute of Applied Astronomy of RAS
|
High resolution maps of BL Lac, 0528+134, 1308+326, 0749+540
have been obtained at 8 GHz for several epochs from 1994 to
1997. Structure evolution on time scales from one month to
several years is discussed.
|
| Possible accelerating motion of IR-mm outbursting sources
in blazars | Short talk
| S. Qian, et al.
| Beijing Astronomical Observatory
|
The interpretation of IR-mm outbursts in blazars are discussed in terms of
shock models. It is shown that in order to explain their rising phases
which preumably occur in the transition between Compton stage and synchrotron
state bulk acceleration of the bursting sources may need to be taken into
account. Spectral tracks Sm(vm) (Sm -turnover flux,
vm - turnover frequency) for non-accelerating and accelerating sources are
compared.
|
| TeV Gamma-ray Flux Variability of Markarian 501 | Short talk
| John Quinn
Whipple Collaboration
|
Markarian 501 was discovered as a source of very high energy
gamma-rays by the Whipple Collaboration in Spring 1995. A
database spanning four years of frequent observations is
now available. The data have been scanned for flux
variability on timescales ranging from years to minutes.
Rapid variability would indicate a relatively
compact emission region and may provide constraints on
emission models. Evidence for significant variability on
timescales as short as a few hours has been found. The
results of this analysis will be presented.
|
| Multiwavelength Observations of BL Lacs:
Implications for Unification | Short Talk
| Travis A. Rector
| CASA - Univ. of Colorado
|
New multi-wavelength observations, including X-ray, optical and
radio (VLA and VLBA) imaging and spectroscopy, for both the EMSS
XBL and 1Jy RBL complete samples will be presented. Many of the
differences between XBLs and RBLs are consistent with the beaming
hypothesis; i.e., BL Lacs are highly-beamed FR-1 radio galaxies,
and XBLs are seen further off-axis than RBLs. But results that I
will present reveal differences in the spatial distributions and
physical properties (e.g., emission-line and extended-radio lumin-
osities) of the two samples, suggesting a more complex picture.
While most XBLs are consistent with being highly-beamed FR-1s, half
of the 1Jy RBLs are more consistent with being beamed high-luminosity
FR-2s. Further, primarily in the 1Jy sample we find evidence for
BL Lacs which are gravitationally-lensed quasars. The implications of
these observations for unification models will be discussed.
|
| EON and blazar monitoring | Poster 24
| Tomas Rezek, Filip Hroch, Rene Hudec
| Institute of Astronomy
|
The European Observation Network (EON) has been established to
coordinate efforts for follow-up observations of Gamma Ray Bursts
(GRBs). There are 12 both professional as well as small observatories
involved in the collaboration recently. The network can be used also for
optical monitoring of selected blazars and AGNs in general, especially
during satellite-related as well as other campaigns. We also report on
optical observations of selected blazars obtained at Brno and Ondrejov
Observatories during 1996-1998.
|
| TeV Gamma-ray observations of Southern BL Lacs with the
CANGAROO 3.8m Telescope | Poster 43
| Michael Roberts
| ICRR (university of Tokyo)
|
abstract: Observational and theoretical results indicate that low-redshift BL Lacertae
objects are the most likelyextragalactic sources to be detectable at TeV energies.
Here we present the results of observations of 7
BL Lacertae objects made between 1993 and 1997. During this period the energy threshold of the 3.8m
telescope was around 2TeV. Searches for steady long-term emission have been made
, and, inspired by the
TeV flares detected from Mkn 421 and Mkn 501, a night-by-night timescale search
has also been performed
for each source. Detailed Monte Carlo simulations are used to estimate upper limits
to both steady and short timescale emission.
|
| Burst Alert Robotic Telescope BART - Monitoring of
selected AGNs | Poster 25
| Jan Soldan (1), Rene Hudec (1), Milos Nemcek (2), Tomas Rezek (
1)
| (1) Institute of Astronomy, Ondrejov, (2) Technical University Ostrava
|
We report on the present status of the Burst Alert Robotic Telescope
(BART) developed at the Astronomical Institute in Ondrejov. The system
is in the final development and test phase and is expected to provide
optical data for different classes of objects: AGNs, blazars,
optical counterparts of Gamma Ray Bursts, cataclysmic variables, supernovae
etc. We address the importance of such system from the point of view of
large number of AGNs and their undersampled light curves.
|
| Optical microvariability observations of southern AGNs | Poster 26
| G.E. Romero, S. Cellone, J.A. Combi
| Instituto Astronomico e Geofisico (Brazil) and IAR (Argentina)
|
We report the first results of an extensive monitoring
program of optical microvariability in southern AGNs. The
observations have been carried out with the 2.15-m CASLEO
telescope, at San Juan, Argentina. This observational
program is aimed to gather variability information of both
southern radio-loud (including several BL-Lac objects) and
radio-quiet AGNs, with temporal resolutions of a few
minutes. Our data, together with those already obtained
by Jang & Miller and Gopal Krishna et al., will provied
the basis for an all-sky statistics of optical
microvariability. The results here presented include
variability information about sources like 0537-441,
0637-751, 1144-379, 2155-304, 2200-181, 2316-42, and others.
|
| Rapid variability of gamma-ray blazars | Short talk
| Salvati,M.(1), Spada,M.(2), Pacini,F.(1,2)
| 1) Osservatorio Astrofisico di Arcetri ;2) Dipartimento di
Astronomia e scienza dello spazio.
|
We present a model for spectral variability of blazars.
The variability is related to anisotropies in the comoving
frame of the jet, associated with the geometry of the shocks
which are responsible for the acceleration of particles.
The model has been used to explain:
1) the ultrarapid burst of the TeV emission of MKN 421 which occurred
on May 15, 1996;
2) the light curves of PKS 2155-304 in the X rays, EUVE, and UV obtained
simultaneously in May 1994.
In the same framework, we shall discuss a possible origin
of Intraday Variability at radio frequencies.
|
| The HST Snapshot Survey of BL Lac Objects Host Galaxy. | Poster 54
| R. Scarpa, C.M. Urry, R. Falomo, A. Treves, J.E. Pesce
| Space telescope Science institute
|
We report the result of the HST sanpshot survey of BL Lac
host galaxies. Almost 100 sources with redshift
ranging from 0.05 to 1.2 were observed. Virtually all source
at z<0.5 are resolved and the host can be studied well
enough that the morphological type is reliably determined.
The BL Lac hosts are, without (or very few) exeption,
bright, giant elliptical with luminosity within a narrow
range of values (-22.3 < M(R) < -25). No significant
cosmological evolution is observed up to z=0.5.
Results are discussed in view of unified models.
|
| Latest X-ray observations of intermediate BL Lac objects | Long talk
| Joachim SIEBERT
| MPE
|
The correlation of the ROSAT All-Sky Survey and the Green
Bank 5GHz radio survey yielded a large number of intermediate
BL Lac objects, a new class of AGN which exhibits spectral
energy distributions intermediate between the previously
known classes of X-ray and radio-selected BL Lac objects. We
report on recent ROSAT, ASCA and BeppoSAX observations of a
few archetypal objects of this class. The obtained broadband
X-ray spectra are crucial for our understanding of this type
of objects and they provide an important test for the
proposed unification scenarios of BL Lac objects.
|
| NOT and HST images of BL Lacs in comparison | Short talk
| A. Sillanpää, et al.
| Tuorla Observatory
|
In this study we will compare images of the BL Lac objects taken with the
HST and the Nordic Optical Telescope. HST images are taken from the
litterature but the NOT images are our own exposures. Both of the
telescopes have almost identical dimensions but the HST is clearly an
"old-type", slow instrument when the NOT is a very fast telescope. This
means that with the HST we can easily reach clearly better angular
resolution but when we are looking at very low surface brightness objects
is the NOT a clearly better instrument. We will show two examples, OJ287
and 2254+074, which clearly show the own advantages of both instruments.
After this comparison we can easily say that we can work a lot in
this field using also modern groud-based telescopes with a good angular
resolution.
|
| Monitoring of the quasar 1156+295 during the
March 1998 outburst | Poster 57
| G. Sobrito, C.M. Raiteri, M. Villata, G. De Francesco, L. Lanteri
| Osservatorio Astronomico di Torino, Italy
|
We present optical data of the quasar 1156+295 (4C 29.45) taken in the last
four years at the Torino Astronomical Observatory with the 1.05 meter REOSC
telescope.
Large-amplitude variations characterize the source light curve; in particular,
during the last observational season an optical outburst was detected,
the R magnitude reaching 13.68 on March 17, 1998.
A CGRO pointing was obtained as a ToO: the comparison between the optical and
gamma-ray emissions will provide constraints for the interpretation of the
high-energy emission mechanisms.
|
| Intensive monitoring of OJ 287 and 3C66A | Short talk
| L.O. Takalo et al.
| Tuorla Observatory
|
We will describe OJ-94 project and the observations of OJ 287 and
3C66A obtained during it. Both object have shown continuous variability
in the optical bands. 3C66A has been in outburst duing the monitoring.
In OJ 287 the predicted optical outbursts occured during November 1994 and
December 1995. In radio bands only the second of these outbursts was seen.
|
| Long term continuum monitoring of AGN with the Metsähovi
and SEST telescopes. | Short Talk
| H. Teräsranta, M. Tornikoski, Metsähovi Radio Research Station
E. Valtaoja, Tuorla Observatory
|
The long term monitoring of a large sample of AGN from 12 to 87 GHz with
the Metsähovi telescope and from 87 to 230 GHz with the SEST telescope is
described. The Metsähovi monitoring started in 1980 and the SEST monitoring in
1988, thus giving decade long records for the most important AGN at millimeter
wavelengths. Our large sample will make it possible to compare different
classes of sources with a fair statistics.
|
| Multifrequency behaviour of PKS 2255-282 | Poster 13
| Merja Tornikoski: Metsähovi Radio Observatory, Finland
Steven Tingay: Jet Propulsion Laboratory, USA
Edward King: CSIRO Office of Space Science and Applications, Australia
David Jauncey: Australia Telescope National Facility, Australia
Anita Muecke, Melanie Johnston, Roger Clay: University of Adelaide, Australia
|
In 1997 we observed a very strong mm-wave outburst in PKS 2255-282,
its flux at 90 GHz increasing 10 Jy. We present our multifrequency
(from 1.4 to 230 GHz) radio flux curves as well as VLBI observations
for this source, and discuss this period of increased multifrequency
activity in PKS 2255-282.
|
| Simultaneous radio/optical outburst in CTA 102 | Poster 14
| Merja Tornikoski, Metsähovi Radio Observatory, Finland
Harri Teräsranta, Metsähovi Radio Observatory, Finland
Thomas J. Balonek, Colgate University, USA
Eli Beckerman, Colgate University (visiting summer student from Wesleyan University), USA
|
In June 1997 CTA 102 had a very strong mm-wave outburst, which was seen
at longer radio wavelengths after a short delay. Optical observations
show an outburst at the same time. We present our radio and optical flux
curves and discuss the nature of this simultaneous radio/optical event.
|
| The Perugia Blazar Monitoring Program | Short Talk
| G. Tosti, M.Fiorucci, M.Luciani
| Astronomical Observatory University of Perugia
|
We here present a summary of the Perugia blazar monitoring
program. To date it is the only ongoing monitoring
program carried out in an automatic way.In three years, we were able
to collect about 12000 BVRcIc photometric points, contributing to
get knowledge on the history of many sources brigther than V=17.0.
|
| The new Robotic Optical Telescope (ORIT) developed at
Perugia Observatory | Poster 27
| Tosti, A. Falchetti-Frescura, M.Fiorucci
| Astronomical Observatory University of Perugia
|
In this paper we present the main characteristics of the
0.80m Optical Robotic Imaging Telescope (ORIT).
ORIT can be considered an improvement of the
Automatic Imaging Telescope which is operating at the Astronomical
Observatory of Perugia since 1994.
It has an Alt-Az mount and sophisticated software
and hardware components which make it a possible candidate
to be the prototype instrument for a world-wide network of robotic telescopes
devoted to intensive monitoring of variable sources.
|
| Optical monitoring of BL Lac during the summer 1997
outburst | Poster 28
| G.Tosti , E. Massaro, L.O. Takalo, M. Villata, et al.
| Astronomical observatory University of Perugia
|
A coordinated optical monitoring of BL Lac
was carried out by the Perugia, Rome, Turin and Tuorla groups, during the
great 1997 outburst to study multiband characteristics of its variability.
The source was observed at the higher level of luminosity ever recordered
over the past 25 years. Large amplitude variantions have been observed
from day-to-day and more faster ones, having amplitudes greater
than 0.5 mag and timescales of a few hours, are often superimposed to the
longer fluctuations.
|
| What is happening in W Com (ON231)? The extraordinary
1998 outburst | Poster 29
| G.Tosti , E. Massaro, L.O. Takalo, M. Villata, Y. Efimov et al.
|
A coordinated optical monitoring of W Com (ON 231)
was carried out by the Perugia, Rome, Turin and Tuorla groups
in the period from March 1994 to March 1997 (Tosti et al., 1998).
During this period WCom was characterized by the
presence of three major outbursts having the observed
maxima in March 1995, February 1996, and January 1997.
The maximum observed value was V=13.61 during the 1996 burst.
In this paper we present optical photometric and polarimetric
data obtained after March 1997 which show that
at the beginning of 1998 W COM started a new outburst.
The observations obtained in April 1998 show that the source reached
a V magnitude of about V=12.5.
The W COM magnitude observed during the maximum observed
in April 1998 is the brightest ever recordered since the photographic
value of 11.6 observed in 1911 by Wolf (1916).
|
| Short Term X-ray Variability of PKS 2155-304:
Cross Correlation Analysis | Short Talk
| Aldo Treves on behalf of a large collaboration
|
PKS 2155-304 is the brightest blazar in X-rays and one of the strongest
and best studied AGNs at these frequencies. It was monitored by the
BeppoSAX for 2.5 days in November 1996 during the satellite Performance
Verification phase. A further, 1.3 days long, BeppoSAX campaign on the
source was accomplished in November 1997, simultaneously with an EGRET
Target of Opportunity observation. In the latter occasion, the source was
found in a very bright X-ray state. Cross-correlation of the X-ray light
curves of 1996 in different energy ranges has shown that the flux at
higher frequencies leads that at lower frequencies. The measured time lag
between the light curves in the 0.1-1.5 keV and 3-10 keV energy intervals
is ~3 hours. In 1997 the lag, if present, is <= 0.5 hours. The results are
compared with those obtained through multiwavelength campaigns in November
1991 and May 1994.
|
| Modelling the millimetre-to-radio flaring behaviour of the
quasar 3C 273 | Short talk
| M. Turler et al.
| Geneva Observatory & INTEGRAL Science Data Centre
|
We present a decomposition of the millimetre-to-radio
emission of 3C 273 into several independent outbursts.
The decomposition is performed by fitting a set of flares
simultaneously to 13 light curves from about 0.3 mm
(1000 GHz) to 10 cm (3 GHz), which contain together more
than 4000 observations.
We show that it is possible to reproduce the detailed
structure of the observed light curves of 3C 273 during
the last 20 years, with about one outburst per year.
This method allows us to isolate individual outbursts
and to derive their evolution as a function of both time
and frequency. Therefore, we can observe the complete
spectral evolution of an outburst.
Finally, we test shock models in relativistic jets (like
those of Marscher & Gear) by comparing the predicted
properties of a flare with the observed properties.
|
| Jet Behaviour of NRAO 190 after 1994 Gamma-Ray Flare | Poster 7
| Yurchenko, A. V., Marchenko, S. G., Marcher, A.
| St. Petersburg State University, Boston University.
|
In August 1994 EGRET has detected a strong gamma-ray
flare (an order of magnitude above quiscent state)
the source of which was identified with NRAO 190.
During 1995-97 we have obtained 6 images of NRAO 190
with VLBA at 1 cm and 7mm which display a fairly bright
core, a weak stationary component and a bright knot
moving outwards with an apparent velocity of about 13c.
The time of birth of that knot may be extrapolated to
the date of gamma-ray flare. We discuss observed
morphological and flux variability in the frame of
shocked relativistic jet model.
|
| BL Lac Objects & Blazars: Past, Present & future | Invited Talk
| M. Urry
| STScI
|
The past 20 years have seen phenomenal progress in our understanding
of BL Lac objects. They form part of the blazar class, which are
radio-loud AGN distinguished by relativistic jets aligned along our
line of sight. Several critical milestones have helped establish
this picture, first proposed at the Pittsburgh meeting 20 years
ago, particularly the EGRET and TeV detections of beamed gamma-ray
emission. The spectral energy distributions are double peaked and
follow a self-similar sequence in luminosity, which can be explained
by electron cooling on ambient photons. This simple paradigm has
yet to be tested, and further questions remain, notably about
physical conditions in blazar jets - the kinetic power, magnetic
energy density, acceleration time scales, proton content, etc. -
and how this energy is transported in the innermost regions. We also
do not know what kinds of jets nature makes, i.e., the relative
number densities of low- (``blue'' BL Lacs) or high-luminosity
(``red'' BL Lacs and FSRQ) blazars. Since blazars are representative
of all radio-loud AGN, their jet properties have broad implications.
Thus we would also like to understand the circumnuclear structure,
especially the details of hot, highly ionized, high velocity gas
on sub-parsec scales, which could play a role in jet dynamics
and could possibly affect the formation of FRI vs. FRII type jets.
The study of blazars should eventually help us understand the
difference between radio-quiet and radio-loud AGN.
|
| Doppler boosting and orientation effects in BL Lacs and
other AGN | Long talk
| Esko Valtaoja
| Tuorla Observatory
|
We have used the observed variability brightness temperatures
of AGN monitored at Metsähovi to derive the Doppler boosting
factors for a large sample of BL Lacs and other classes of
compact radio sources. A comparison between the boosting
factors derived by this new method and values derived by
traditional methods (e.g., synchrotron self-Compton and
equipartition estimates) will be presented. The variability
Doppler boosting factors and the VLBI expansion speeds are
used to derive the viewing angles and the Lorentz factors of
the sources. A comparison of these parameters for BL Lacs and
for other AGN will be presented.
|
| A helical-jet model for Mkn 501 | Poster 38
| M. Villata and C.M. Raiteri
| Osservatorio Astronomico di Torino
|
We present a geometrical model for the interpretation of the
Mkn 501 spectral energy distribution (SED) variations.
It is shown how a helical jet with a steady emission
can explain the SEDs constructed with
simultaneous data taken when the source was in different brightness states:
in particular, it can account fairly well for the exceptionally high X-ray
luminosity state recently detected by the BeppoSAX satellite.
|
| Blazar Activity Across the Electromagnetic Spectrum | Invited Talk
| Stefan J. Wagner
| LSW, Heidelberg, and MSSSO
|
During the past decade the parameter space accessible for blazar studies
increased markedly. In the regime of temporal coverage fast variability
(IDV) on scales of days, hours and minutes was established. In frequency
space the 100 keV - 10 TeV regime (8 orders of magnitude) opened up.
It is clear now that a full understanding of the physical processes
requires variability studies which have dense coverage in time as well
as in frequency. While correlations between variations in different parts
of the spectrum could be established, it also became clear that the
relationship is likely to be more complicated than assumed previously.
Recent empirical progress as well as the state of the art of theoretical
modelling shall be reviewed.
|
| Variability from GHz to GeV: S5 0716+714 in 1994 and 1996 | Poster 30
| S.J. Wagner on behalf of the 0716-collaboration
|
0716+714 is one of the most enigmatic Blazars. It is not only one of
the most active sources, regularly displaying IDV in radio and optical
bands. Its spectrum is also entirely dominated by non-thermal emission
throughout the entire energy range, and it has also always been detected
when observed with EGRET within the sensitive part of its field-of-view.
The high duty cycle led to a number of multi-frequency campaigns.
In particular, large campaigns have been carried out in the spring
periods of 1994 and 1996. The source was active in all wavebands observed.
The results of these campaigns will be presented and discussed.
|
| The Dual Jet Model for Blazar Continuum Emision | Poster 37
| J. R. Webb, E. Benitez, E. Howard & B. Punsley
| Florida International University
|
We address the current state of understanding of the
Blazar phenomena and present a relatively new and
interesting model for explaining the dicotomy of
blazar emission characteristics. This new model, which
was investigated in a series of papers by
Brian Punsley (Punsley, 1997) is capable of explaining many
features exhibited by Blazars, inclunding the radio and
gamma-ray properties of OVV quasars and BL Lac sources.
We also present new evidence supporting this
''dual jet'' model and some ideas for further testing.
|
| Spectral and Temporal Variability Incorporating
General Relativistic Effects | Long talk
| Paul J. Wiita and Gang Bao
| Georgia State University, Atlanta, GA 30303, USA
|
Composite spectra, including contributions from an accretion
disk around a black hole, a primary X-ray power-law
source, and the photons reflected from the disk, are
considered in Schwarzschild geometry. The strength, shape and
broadening of the reflected spectrum depend on the directions
of the X-ray source relative to the disk and the observer's viewing
angle. The reflection hump can essentially disappear if viewed
far from the symmetry axis as the X-ray photons are affected
by gravity both approaching and leaving the disk. As the innermost
part of accretion disks are expected to be unstable and clumpy,
some of the X-ray variability can be due to motions of these regions
orbiting the black hole. Even for face-on geometries, rotation
induced variability can be observed as long as the non-thermal
source is not exactly on the symmetry axis.
|
| Red synchrotron jets in Parkes Quasars | Short Talk
| Matthew Whiting
| University of Melbourne
|
Using a new set of observations of Parkes Quasars, I have
fitted new models of synchrotron emission to broadband
spectra in the optical and near-infrared. I present these
results, showing that a majority of the sources in the sample
show strong evidence for a red optical/infrared synchrotron
component over the top of the underlying quasar continuum,
which would produce a reddened spectrum. I will also present
measurements of emission line equivalent widths that support
the synchrotron hypothesis. In addition, some sources show
evidence for reddening by a dust component as well as the
synchrotron component, and can be better fitted by a three
component model.
|
| The REX survey: how to find a large and complete
sample of BL Lacs | Short talk
| A. Wolter
| Osservatorio Astronomico di Brera
|
The number of known BL Lacs is not large, if compared to that of other classes
of AGN, and in particular the statistically complete samples are made of a few
tens of objects. This fact makes any quantitative derivation of the global
properties of the class subject to large uncertainties.
We have started a project to find a sizable, complete and deep sample of Radio
Emitting X-ray sources (REXs) using the available data from a VLA survey (NVSS)
and the ROSAT PSPC archive. We expect that the sample will contain, by the end
of the identification process, about 200 BL Lacs, enabling us to better study
their statistical properties, like the X-ray, radio and optical luminosity
functions and their cosmological evolution, and to address the apparent dichotomy
of RBL and XBL using a unique sample to test the two classes at the same time.
We will describe the project and its current status.
|
| A multiwavelength study of BL Lac Mg II absorbers | Poster 55
| Emily Xanthopoulos
| NRAL Jodrell Bank
|
Mg II absorption systems have been thought by several authors to arise in halos and
disks of large intervening galaxies. A generic difference between the galaxies
that are known to cause Mg II absorption in the line of sight to a distant quasar, and
those that do not, is that almost all of the Mg II absorbers show strong emission lines.
A total of 10 Mg II systems have been identified in the 1 Jy radio-selected BL L
ac sample with five of the ten systems having an equivalent width larger than 1 A.
This excess of Mg II absorbers in BL Lac objects (4-5 greater than the number expected
based upon quasar sight lines and 2.5-3 sigma greater than the expectation value)
could possibly be explained either by the fact that the Mg II absorbers are intrinsic
to the BL Lac (4 of these systems do not have emission-line redshifts)
or that there is a correlation between the absorbing gas in the foreground and the
nearly featureless spectra of these BL Lac objects compared to quasars. Such correlation
can be created by gravitational microlensing (characteristics of the BL Lac objects
are thought to be created by this microlensing effect and some of the BL Lac
objects might be distant quasars brightened by this effect).
Here we present a multiwavelegth study of these 10 systems. Comparisons between
the Mg II BL Lac absorbers that show larger than 1 A equivalent width and those that
do not, as well as between the whole sample of 10 Mg II BL Lac absorbers
with BL Lacs that do not show at all Mg II absorbers and a sample
of quasars are also presented.
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