Quasar Black Hole Mass Estimates in the Era of Time Domain Astronomy

TL;DR

This study explores how the high-frequency variability of X-ray and optical emissions in active galactic nuclei correlates with black hole mass, introducing a new statistical method for analyzing lightcurves to improve mass estimation accuracy.

Contribution

We develop a novel statistical technique to estimate power spectral densities from photon count lightcurves without binning, enabling more precise black hole mass estimates from variability data.

Findings

High-frequency X-ray PSD normalization inversely correlates with black hole mass.

Optical variability amplitude is anti-correlated with black hole mass and luminosity.

New method allows black hole mass estimation with ~0.38 dex precision from X-ray variability.

Abstract

We investigate the dependence of the normalization of the high-frequency part of the X-ray and optical power spectral densities (PSD) on black hole mass for a sample of 39 active galactic nuclei (AGN) with black hole masses estimated from reverberation mapping or dynamical modeling. We obtained new Swift observations of PG 1426+015, which has the largest estimated black hole mass of the AGN in our sample. We develop a novel statistical method to estimate the PSD from a lightcurve of photon counts with arbitrary sampling, eliminating the need to bin a lightcurve to achieve Gaussian statistics, and we use this technique to estimate the X-ray variability parameters for the faint AGN in our sample. We find that the normalization of the high-frequency X-ray PSD is inversely proportional to black hole mass. We discuss how to use this scaling relationship to obtain black hole mass estimates from the short time-scale X-ray variability amplitude with precision ~ 0.38 dex. The amplitude of optical variability on time scales of days is also anti-correlated with black hole mass, but with larger scatter. Instead, the optical variability amplitude exhibits the strongest anti-correlation with luminosity. We conclude with a discussion of the implications of our results for estimating black hole mass from the amplitude of AGN variability.