Other
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt.........6b&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, LOS ANGELES, 1996.Source: Dissertation Abstracts International, Volume: 57-06, Sectio
Other
Accretion Disks, Seyfert Galaxies
Scientific paper
The six month ROSAT All-Sky Survey in 1990-91 provided an unprecedented opportunity to study the soft X-ray variability of a large sample of active galactic nuclei (AGN) on timescales of days to months. Seventy -four AGN within 15 deg of the ecliptic poles were monitored by ROSAT during the survey. In addition, coordinated monitoring campaigns were performed in the UV, optical, and radio wavebands. These observations have been used to study the amplitudes and timescales of variability in the soft X-ray and lower energy wavebands and to search for correlated variations and lags between the wavebands to test models for the origin of the soft X-ray excess in thermal AGN; in particular, models which explain the soft X-ray excess as the high-frequency tail of the UV excess. X-ray variability was detected in 30% of the 37 light curves analyzed. This percentage is a lower limit, since most sources were weaker than the background. The sources varied by ~8-80% (rms) in the X-rays over baselines of a few days to six months. Radio -quiet quasars showed smaller X-ray variability amplitudes (_sp{~}<10%) than the other AGN classes. The X-ray power spectra of the blazars were at least as steep as random walk over timescales of ~30 days, while the radio-quiet AGN showed flicker noise power spectra over timescales of up to 10 days. No correlation was found between variability timescales and luminosity. The fractional rms variability amplitude (sigma x/Lx) for the radio-quiet AGN was found to correlate with X-ray luminosity as L_sp{x}{-0.5}. . Optical light curves taken in various filters were well correlated in the thermal AGN. For 1803 + 676, the allowed crossing time was too short (_sp {~}<20 days) for a physical disturbance to propagate through the disk (v _sp{~}> 0.07c). This implies that the optical variability must result from photon reprocessing of higher energy photons (i.e., X-rays or UV). Energetics arguments indicate that X-ray reprocessing cannot produce the observed UV variability in 1803 + 676. Since the X-ray and optical light curves for this source were not correlated, we conclude that the optical variability is probably due to reprocessed UV emission, not reprocessed X-rays.
No associations
LandOfFree
Multiwavelength Variability of Active Galactic Nuclei on Intermediate Timescales does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Multiwavelength Variability of Active Galactic Nuclei on Intermediate Timescales, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multiwavelength Variability of Active Galactic Nuclei on Intermediate Timescales will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1129326