Blazar Times - No. 46 - August 2002
The Blazar Times
A Research Newsletter Dedicated to the BL Lac and Blazar Phenomena
No. 46 - August 2002 Editor: Travis A. Rector (


Journal Abstracts 1

Abstract Guidelines 4

Journal Abstracts

Broad-band continuum and line emission of the g-ray blazar PKS 0537-441

E. Pian1, R. Falomo2, R. C. Hartman3, L. Maraschi4, F. Tavecchio4, M. Tornikoski5, A. Treves6, C. M. Urry7, L. Ballo4, R. Mukherjee8, R. Scarpa9, D. J. Thompson3, J. E. Pesce10

1 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste, Italy
2 Osservatorio Astronomico di Padova, via dell'Osservatorio 5, I-35122 Padova, Italy
3 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
4 Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy
5 Metsahovi Radio Observatory, Metsahovintie 114, FIN-02540 Kylmala, Finland
6 Dipartimento di Scienze, University of Insubria, Via Valleggio 11, I-22100 Como, Italy
7 Department of Physics, Yale University, P.O. Box 208121, New Haven CT 06520-8121

8 Department of Physics and Astronomy, Barnard College, Columbia University, New York, NY 10027
9 European Southern Observatory, 3107 Alonso de Cordova, Santiago, Chile
10 Eureka Scientific and George Mason University, 4400 University Drive, Fairfax, Virginia, VA 22030

PKS 0537-441, a bright g-ray emitting blazar, was observed at radio, optical, UV and X-ray frequencies during various EGRET pointings, often quasi-simultaneously. In 1995 the object was found in an intense emission state at all wavelengths. BeppoSAX observations made in 1998, non-simultaneously with exposures at other frequencies, allow us to characterize precisely the spectral shape of the high energy blazar component, which we attribute to inverse Compton scattering. The optical-to-g-ray spectral energy distributions at the different epochs show that the g-ray luminosity dominates the bolometric output. This, together with the presence of optical and UV line emission, suggests that, besides the synchrotron self-Compton mechanism, the Compton upscattering of photons external to the jet (e.g., in the broad line region) may have a significant role for high energy radiation. The multiwavelength variability can be reproduced by changes of the plasma bulk Lorentz factor. The spectrum secured by IUE in 1995 appears to be partially absorbed shortward of ~ 1700 Å. However, this signature is not detected in the HST spectrum taken during a lower state of the source. The presence of intervening absorbers is not supported by optical imaging and spectroscopy of the field.

In press in A&A

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The Accretion Rates and Spectral Energy Distributions of BL Lacertae Objects

Jian-Min Wang1,2, Rüdiger Staubert2  and Luis C. Ho3

1 Institut für Astronomie und Astrophysik, Abt. Astronomie, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany;
2 Laboratory for High-Energy Astrophysics, Institute of High-Energy Physics, CAS, Beijing 100039, P. R. China.
3 The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101-1292.

We investigate the relationship between accretion rates and the spectral energy distributions (SEDs) of BL Lac objects, using a sample of objects for which published information on the host galaxies, emission-line luminosities, and peak frequencies and luminosities of their SEDs are available. The sample is composed of 43 BL Lac objects which have a relatively continuous distribution of peak frequencies. Under the assumption that the observed emission lines are photoionized by the central accretion disk, we use the line luminosities to estimate the accretion luminosities and hence accretion rates. We find that low frequency-peaked BL Lac objects (LBLs) span a wide range of accretion rates, whereas high frequency-peaked BL Lac objects (HBLs) cover a more restricted range of lower values. There appears to be a continuous distribution of accretion rates between the two subclasses of BL Lac objects. We find that the peak frequency of the SED, npk, correlates with the accretion rate, approximately with the form npk L-3 in HBLs and npk L-0.25 in LBLs, where L Llines/c2. The peak luminosity of the SED is also correlated with L. These results suggest that the accretion rate influences the shape of the SED in BL Lac objects. They also support models which couple the jet and the accretion disk. We present a physical scenario to account for the empirical trends.

2002, ApJ, 579, November 10

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Time Dependent Modeling of the Markarian 501 X-ray and TeV Gamma-Ray Data Taken During March and April, 1997

H. Krawczynski 1,2, Paolo Coppi1  and Felix Aharonian3

1 Yale University, P.O. Box 208101, New Haven, CT 06520-8101, USA
2 Now at: Washington University in St. Louis, Phys. Dept., 1 Brookings Drive, Campus Box 1105, St. Louis, MO 63130, USA
3 Max Planck Institut für Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany

If the high-energy emission from TeV blazars is produced by the Synchrotron Self-Compton (SSC) mechanism, then simultaneous X-ray and gamma-ray observations of these objects are a powerful probe of the electron (and positron) populations responsible for this emission. Understanding the emitting particle distributions and their temporal evolution in turn allows us to probe physical conditions in the inner blazar jet and test, for example, various acceleration scenarios. Furthermore, by constraining the SSC emission model parameters, such observations enable us to predict the intrinsic (unabsorbed) gamma-ray energy spectra of these sources, a major uncertainty in current attempts to use gamma-ray observations to constrain the intensity of the Diffuse Extragalactic Background Radiation (DEBRA) at optical/infrared wavelengths. As a next step in testing the SSC model and as a demonstration of the potential power of coordinated X-ray and gamma-ray observations, we model in detail the X-ray and gamma-ray light curves of the TeV blazar Mrk 501 during its April-May 1997 outburst with a time dependent SSC model. Extensive, quasi-simultaneous X-ray and gamma-ray coverage exists for this period. We discuss and explore quantitatively several of the flare scenarios presented in the literature. We show that simple two-component models (with a soft, steady X-ray component plus a variable SSC component) involving substantial pre-acceleration of electrons to Lorentz factors on the order of gmin  = 105 describe the data train surprisingly well. All considered models imply an emission region that is strongly out of equipartition and low radiative efficiencies (ratio between kinetic jet luminosity and comoving radiative luminosity) of 1 per-mill and less. Degeneracy in both, model variant and jet parameters, prevents us to use the time resolved SSC calculations to substantially tighten the constrains on the amount of extragalactic gamma-ray extinction by the DEBRA in the relevant 0.5-50 microns wavelength range, compared to earlier work.

Accepted by MNRAS

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The Guaranteed Gamma Ray Background

Vasiliki Pavlidou1 and Brian D. Fields1

1 Center for Theoretical Astrophysics, Department of Astronomy, University of Illinois, Urbana, IL 61801 USA

The diffuse extragalactic gamma-ray background (EGRB) above 100 MeV encodes unique information about high-energy processes in the universe. Numerous sources for the EGRB have been proposed, but the two systems which are certain to make some contribution are active galaxies (blazars) as well as normal galaxies. In this paper, we evaluate the contribution to the background from both sources. The active galaxy contribution arises from unresolved blazars. We compute this contribution using the Stecker-Salamon model. For normal galaxies, the emission is due to cosmic-ray interactions with diffuse gas. Our key assumption here is that the cosmic-ray flux in a galaxy is proportional to the supernova rate and thus the massive star formation rate, quantified observationally by the cosmic star formation rate (CSFR). In addition, the existence of stars today requires a considerably higher ISM mass in the past. Using the CSFR to compute both these effects, we find that normal galaxies are responsible for a significant portion ( ~ 1/3) of the EGRB near 1 GeV, but make a smaller contribution at other energies. Finally, we present a ``minimal'' two-component model which includes contributions from both normal galaxies and blazars. We show that the spectrum of the diffuse radiation is a key constraint on this model: while neither the blazar spectra, nor the galactic spectra, are separately optimal fits to the observed spectrum, the combined emission provides an excellent fit. We close by noting key observational tests of this two-component model, which can be probed by future gamma-ray observatories such as GLAST.

Reference: 2002 ApJL, 575, 5-8

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Correlated Radio and Optical Variations in a Sample of AGN

Hanski, M. T.1, Takalo, L. O.1  and Valtaoja, E.1,2

1 Tuorla Observatory, Väisäläntie 20, FIN-21500 Piikkiö, Finland
2 Department of Physics, 20014 University of Turku, Finland

Radio light curves of 20 sources from the Metsähovi monitoring program are compared with optical light curves collected from the literature. The Discrete Correlation Function analysis is applied to the data sets. A new qualitative method to study correlations is introduced, where the radio light curves are replaced by model light curves consisting of exponential outbursts. The optical flux level is compared to the phase and flux level of the model flares. Seven sources show clear correlations using the DCF analysis with six more showing a possible correlation between optical and radio events with time lags from zero to several hundred days. For twelve sources at least one simultaneous optical and radio event is seen. For eleven sources, when comparing optical flux levels with the phase of the model radio outbursts, the optical flux levels were high at the peak of the model radio outbursts. For sixteen sources, when comparing the optical flux level with the radio model flare flux level, the optical flux level was high when radio flux level was high, as well.

Accepted by A&A

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Abstract Guidelines

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