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High-Energy Continuum Variability in Active Galactic Nuclei

Published online by Cambridge University Press:  19 July 2016

Rick Edelson
Rick Edelson*
Affiliation:
Department of Physics and Astronomy; University of Iowa; Iowa City, IA 52242; U.S.A.

Abstract

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CGRO and IUE observations suggest that the strong, aperiodic variability seen in the Exosat long-look observations of AGN extends over a much wider energy band. Some BL Lac objects (but no Seyfert 1 galaxies) have shown X-ray variations which were so rapid that they violate the assumptions of isotropy inherent in the Eddington limit. In the ultraviolet, Seyfert 1s as a class show an anti-correlation between the variability amplitude and luminosity, while BL Lacs show a positive correlation. Furthermore, Seyfert 1s show strong flux-correlated spectral variability, while BL Lacs show little or none. All of this suggests that the high-energy continua of BL Lacs are beamed towards us, while the ultraviolet continua of Seyfert 1s are emitted isotropically.

The November 1991 multi-waveband monitoring of the BL Lac PKS 2155−304 showed strong correlated variability, with the soft X-rays leading the ultraviolet by a few hours, and no measurable lag between the ultraviolet and optical down to a limit of ≲ 1.5 hr. This indicates that the X-rays from this BL Lac are not produced by Compton upscattering, and that the ultraviolet does not come directly from a thermal source such as an accretion disk. This also strongly constrains the relativistic jet model, suggesting that all of the radiation is produced in a flattened region like a shock front.

Low temporal resolution ultraviolet/optical monitoring of the Seyfert 1 NGC 5548 in 1989 yielded a strong correlation with no measurable lag to a limit of ≲4 days, casting some doubt on the standard model of thermal emission from an accretion disk in Seyfert 1s. Upcoming X-ray/ultraviolet/optical monitoring of the Seyfert 1 NGC 4151 in December 1993 will have much faster sampling, to permit a strong test of both this model and the competing reprocessing model.

Keywords

BL Lacertae Objectsγ-RaysQuasarsSeyfert GalaxiesUltravioletVariabilityX-Rays
Type
Variability
Information
Symposium - International Astronomical Union , Volume 159: Multi-Wavelength Continuum Emission of AGN , 1994 , pp. 113 - 122
Copyright
Copyright © Kluwer 1994 

References

Abramowicz, M. & Nobili, L. 1982, Nature, 300, 506 Google Scholar
Barvainas, R. 1993, ApJ, 421, 513 Google Scholar
Barr, P. 1986, MNRAS, 223, 29p Google Scholar
Brinkmann, W. et al. 1993, ApJ, in preparation Google Scholar
Bregman, J. 1991, Astron. Astrophys. Rev., 2, 125 CrossRefGoogle Scholar
Bond, I., Matsuoka, M. & Yamanuchi, M. 1993, ApJ, 405, 179 Google Scholar
Cavallo, G. & Rees, M. J. 1978, MNRAS, 183, 359 CrossRefGoogle Scholar
Clavel, J. et al. 1991, ApJ, 366, 64 Google Scholar
Clavel, J. et al. 1992, ApJ, 393, 113 CrossRefGoogle Scholar
Czerny, B. & Elvis, M. 1987, ApJ, 321, 305 CrossRefGoogle Scholar
Done, C. et al. 1990, MNRAS, 243, 713 Google Scholar
Edelson, R., Krolik, J. & Pike, G. 1990, ApJ, 359, 86 CrossRefGoogle Scholar
Edelson, R. 1992, ApJ, 401, 516 CrossRefGoogle Scholar
Edelson, R. et al. 1993, ApJ, submitted Google Scholar
Fabian, A. C. 1979, Proc. R. Soc. Lond., 366, 449 Google Scholar
Fiore, F. et al. 1992, AA, 262, 37 Google Scholar
Ghisellini, G., Maraschi, L. & Treves, A. 1985, AA, 146, 204 Google Scholar
Kniffen, D. et al. 1993, ApJ, 411, 133 CrossRefGoogle Scholar
Krolik, J., Horne, K., Kallman, T., Malkan, M., Edelson, R. & Kriss, G. 1991, ApJ, 371, 541 CrossRefGoogle Scholar
Laor, A. & Netzer, H. 1989, MNRAS, 238, 897 Google Scholar
Madau, P. 1988, ApJ, 327, 116 Google Scholar
Madejski, G. et al. 1993, Nature, in press Google Scholar
Maisack, M. et al. 1993, ApJL, 407, L61 Google Scholar
Marscher, A. & Gear, W. 1985, ApJ, 298, 114 CrossRefGoogle Scholar
McHardy, I. & Czerny, B. 1985, Nature, 325, 696 CrossRefGoogle Scholar
Mushotzky, R., Pounds, K. & Done, C. 1993, Ann. Rev. Astron. Ap., 31, in press CrossRefGoogle Scholar
Nandra, P. et al. 1990, MNRAS, 242, 660 Google Scholar
Nandra, P. et al. 1991, MNRAS, 248, 760 CrossRefGoogle Scholar
Paciesas, W. et al. 1993, in Proceedings of the Compton γ-Ray Observatory Symposium , ed. Gehrels, N., (St. Louis, MO), in press Google Scholar
Pearson, T. et al. 1981, Nature, 290, 365 CrossRefGoogle Scholar
Perola, G. C. et al. 1986, ApJ, 306, 508 Google Scholar
Peterson, B. et al. 1991, ApJ, 368, 119 Google Scholar
Press, W., Rybicki, G., & Hewitt, J. 1992, ApJ, 385, 404 Google Scholar
Rees, M. 1984, Ann. Rev. Astron. Ap., 22, 471 Google Scholar
Reichert, G. et al. 1993, ApJ, in press Google Scholar
Sun, W.-S. & Malkan, M. A. 1989, ApJ, 346, 68 CrossRefGoogle Scholar
Tagliaferri, G., Stella, L., Maraschi, L., Treves, A., & Celotti, A. 1991, ApJ, 380, 78 Google Scholar
Urry, C. M. et al. 1993, ApJ, 411, 614 Google Scholar
Wandel, A. & Petrosian, V. 1988, ApJL, 329, L11 Google Scholar
Wandel, A. & Urry, C. M. 1991, ApJ, 367, 78 Google Scholar
Yaqoob, T. & Warwick, R. S. 1991, MNRAS, 248, 773 Google Scholar