Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001apj...555..775c&link_type=abstract
The Astrophysical Journal, Volume 555, Issue 2, pp. 775-785.
Astronomy and Astrophysics
Astronomy
50
Accretion, Accretion Disks, Galaxies: Active, Ultraviolet: Galaxies, X-Rays: Galaxies
Scientific paper
The UV/optical light curves of active galactic nuclei (AGNs) may be roughly characterized by aperiodic fractional flux variations of ~10% on timescales of 1 month. The physical mechanism(s) responsible remain ill defined. We present a structure function analysis, i.e., measure the power distribution over a range of timescales τ, of 13 AGNs to constrain the origin of UV/optical emission. On timescales τ~5-60 days, the mean UV and optical power-density spectra (PDS) are equivalent. This may suggest that the underlying energy generating mechanism is identical. The combined UV/optical PDS is P(f)~f-α with α=2.13+0.22-0.06. For sources with measured X-ray PDS indices, we find they are indistinguishable from their UV/optical counterparts. This supports scenarios whereby X-rays are generated via Compton upscattering of UV photons, to later radiatively drive optical variations. At the same time, we present evidence for characteristic variability timescales τchar of ~5-100 days in 10 sources. These variability timescales combined with reverberation based masses M suggest a M-τchar relationship; higher mass systems have larger characteristic timescales. The UV τchar may possibly reflect dynamical or accretion disk thermal timescales. We find suggestive evidence for a dichotomy, at τ~30 days and M~107 Msolar, between short- and long-timescale optical variations. These optical variations may be attributable to dynamical and accretion disk thermal or starburst activity timescales, respectively.
Collier Stefan
Peterson Bradley. M.
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