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Multiwavelength monitoring of active galactic nucleiRecent multiwavelength monitoring of active galactic nuclei (AGN), particularly with the IUE satellite, has produced extraordinay advances in our understanding of the energy-generation mechanism(s) in the central engine and of the structure of the surrounding material. Examples discussed here include both ordinary AGN and blazars (the collective name for highly variable, radio-loud AGN like BL Lac objects and Optically Violently Variable quasars). In the last decade, efforts to obtain single-epoch multiwavelength spectra led to fundamentally new models for the structure of AGN, involving accretion disks for AGN and relativistic jets for blazars. Recent extensions of multiwavelength spectroscopy into the temporal domain have shown that while these general pictures may be correct, the details were probably wrong. Campaigns to monitor Seyfert 1 galaxies like NGC 4151, NGC 5548 and Fairall 9 at infrared, optical, ultraviolet and X-ray wavelengths indicate that broad-emission line regions are stratified by ionization, density, and velocity; argue against a standard thin accretion disk model; and suggest that X-rays represent primary rather than reprocessed radiation. For blazars, years of radio monitoring indicated emission from an inhomogeneous synchrotron-emitting plasma, which could also produce at least some of the shorter-wavelength emission. The recent month-long campaign to observe the BL Lac object PKS 2155-304 has revealed remarkably rapid variability that extends from the infrared through the X-ray with similar amplitude and little or no discernible lag. This lends strong support to relativistic jet models and rules out the proposed accretion disk model for the ultraviolet-X-ray continuum.
Document ID
19950049986
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Urry, C. M.
(Space Telescope Science Inst. Baltimore, MD, US, United States)
Date Acquired
August 16, 2013
Publication Date
December 1, 1993
Publication Information
Publication: Advances in Space Research
Volume: 13
Issue: 12
ISSN: 0273-1177
Subject Category
Astrophysics
Accession Number
95A81585
Funding Number(s)
CONTRACT_GRANT: NAG5-1034
Distribution Limits
Public
Copyright
Other

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