Modeling the UV/optical continuum time-lags in AGN

TL;DR

This study demonstrates that thermal reverberation in standard accretion discs can accurately explain the UV/optical continuum time-lags observed in active galactic nuclei, and proposes a method to estimate black hole spin from these lags.

Contribution

The paper introduces an analytic model for UV/optical time-lags in AGN and applies it to observational data, confirming its effectiveness and exploring black hole spin estimation.

Findings

Thermal reverberation explains UV/optical time-lags in all studied Seyferts.

Observed time-lags match predictions for standard accretion discs with known black hole parameters.

Model under-predicts X-ray to UV lags, likely due to broad X-ray auto-correlation functions.

Abstract

Thermal reverberation in accretion discs of active galactic nuclei is thought to be the reason of the continuum UV/optical time lags seen in these sources. Recently, we studied thermal reverberation of a standard Novikov-Thorne accretion disc illuminated by an X-ray point-like source, and we derived an analytic prescription for the time lags as function of wavelength. In this work, we use this analytic function to fit the time-lags spectra of seven Seyferts, that have been intensively monitored, in many wave-bands, in the last few years. We find that thermal reverberation can explain the observed UV/optical time lags in all these sources. Contrary to previous claims, the magnitude of the observed UV/optical time-lags is exactly as expected in the case of a standard accretion disc in the lamp-post geometry, given the black hole mass and the accretion rate estimates for the objects we study. We derive estimates of the disc accretion rates and corona height for a non-spinning and a maximally spinning black hole scenarios. We also find that the modelling of the continuum optical/UV time-lags can be used to estimate the black hole spin, when combined with additional information. We also find that the model under-predicts the observed X-ray to UV time-lags, but this difference is probably due to the broad X-ray auto-correlation function of these sources.