Accretion Disk Size Measurements of Active Galactic Nuclei Monitored by the Zwicky Transient Facility

Kavli Affiliate: Luis C. Ho

| First 5 Authors: Wei-Jian Guo, Yan-Rong Li, Zhi-Xiang Zhang, Luis C. Ho, Jian-Min Wan

| Summary:

We compile a sample of 92 active galactic nuclei (AGNs) at z<0.75 with $gri$
photometric light curves from the archival data of the Zwicky Transient
Facility and measure the accretion disk sizes via continuum reverberation
mapping. We employ Monte Carlo simulation tests to assess the influences of
data sampling and broad emission lines and select out the sample with
adequately high sampling cadences (3 days apart in average) and minimum
contaminations of broad emission lines. The inter-band time delays of
individual AGNs are calculated using the interpolated cross-correlation
function and then these delays are fitted with a generalized accretion disk
model, in which inter-band time delays are a power function of wavelength,
black hole mass, and luminosity. A Markov-chain Monte Carlo method is adopted
to determine the best parameter values. Overall the inter-band time delays can
be fitted with the $tau propto lambda^{4/3}$ relation as predicted from a
steady-state, optically thick, geometrically thin accretion disk, however, the
yielded disk size is systematically larger than expected, although the ratio of
the measured to theoretical disk sizes depend on using the emissivity — or
responsivity — weighted disk radius. These results are broadly consistent with
previous studies, all together raising a puzzle about the "standard" accretion
disk model.

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