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

TL;DR

This study measures accretion disk sizes in 92 active galactic nuclei using reverberation mapping from Zwicky Transient Facility data, finding larger-than-expected disk sizes consistent with prior research, challenging standard models.

Contribution

It provides a large sample analysis of AGN disk sizes with improved sampling and modeling, revealing systematic discrepancies with the standard accretion disk theory.

Findings

Inter-band time delays follow the $ au \\propto \\lambda^{4/3}$ relation.

Measured disk sizes are systematically larger than theoretical predictions.

Results support previous findings of deviations from the standard accretion disk model.

Abstract

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.