Characteristic Ultraviolet/Optical Timescales in Active Galactic Nuclei

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 τ_char of ~5-100 days in 10 sources. These variability timescales combined with reverberation based masses M suggest a M-τ_char relationship; higher mass systems have larger characteristic timescales. The UV τ_char may possibly reflect dynamical or accretion disk thermal timescales. We find suggestive evidence for a dichotomy, at τ~30 days and M~10⁷ M_solar, between short- and long-timescale optical variations. These optical variations may be attributable to dynamical and accretion disk thermal or starburst activity timescales, respectively.