Kavli Affiliate: Luis C. Ho
| First 5 Authors: Suyeon Son, Minjin Kim, Luis C. Ho, ,
| Summary:
The characteristic timescale ($tau$) of continuum variability of the
accretion disk in active galactic nuclei (AGNs) is known to be related to the
thermal timescale, which is predicted to scale with AGN luminosity ($L$) and
restframe wavelength ($lambda_{rm RF}$) as $t_{rm th} propto L^{0.5}
lambda_{rm RF}^2$ in the standard disk model. Using multi-epoch spectroscopic
data from the Sloan Digital Sky Survey Reverberation Mapping project, we
construct ultraviolet ensemble structure functions of luminous AGNs as a
function of their luminosity and wavelength. Assuming that AGNs exhibit a
single universal structure function when $Delta t$ is normalized by $tau$,
wherein $tau propto L^{a} lambda_{rm RF}^{b}$, we find $a=0.50pm0.03$ and
$b=1.42pm0.09$. While the value of $a$ aligns with the prediction from the
standard disk model, $b$ is significantly smaller than expected, suggesting
that the radial temperature (color) profile of the accretion disk is
significantly steeper (shallower) than the standard disk model. Notably, this
discrepancy with theory has been observed in previous studies based on
spectroscopic reverberation mapping and gravitational microlensing. Although no
current model of accretion disks fully matches our results, our findings
provide valuable constraints for testing future physical models.
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