ABSTRACTS

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VLBI Studies of Mrk 501 and 0814+425 Short talk
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.