ARlogo Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
Copyright © 1997 by Annual Reviews. All rights reserved

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

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VARIABILITY OF ACTIVE GALACTIC NUCLEI

Marie-Helene Ulrich

European Southern Observatory, Karl-Schwarzschild Strasse 2, D-85748 Garching bei München, FRG

Laura Maraschi

Brera Astronomical Observatory, via Brera 28, 20121 Milan, Italy

C. Megan Urry

Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA


ABSTRACT. A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X ray source. The validity of this model for high luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The "accretion disk plus wind" model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined.

In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The general correlation found between variations in TeV gamma rays and in X rays for Mrk 421, and between variations in GeV gamma rays and in the IR-optical-UV bands for 3C 279, two prototype objects, supports models in which the same population of relativistic electrons radiates the low frequency continuum via synchrotron and the high frequency continuum via inverse Compton scattering of soft photons. Identifying the dominant source of soft photons, which is at present unclear, will strongly constrain the jet physics.


Key words: Seyfert galaxies, quasars, blazars, black holes, emission lines, accretion disks, jets


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