Revisiting UV/optical continuum time lags in AGN

TL;DR

This paper updates a model for UV/optical continuum time lags in AGN, incorporating variable black hole spins and colour correction factors, and fits it to observational data from four Seyfert galaxies to understand accretion and corona properties.

Contribution

The model is extended to include any black hole spin and colour correction, and applied to multiple AGN to better interpret observed time lags and accretion physics.

Findings

Model fits well to observed time-lag spectra in four Seyfert galaxies.

Detected significant U-band delay excess in NGC 4593 due to BLR emission.

Provided an updated analytic expression for non-accretion powered X-ray sources.

Abstract

In this paper, we present an updated version of our model (KYNXiltr) which considers thermal reverberation of a standard Novikov-Thorne accretion disc illuminated by an X-ray point-like source. Previously, the model considered only two cases of black hole spins, and assumed a colour correction factor $f_{\rm col} = 2.4$. Now, we extend the model to any spin value and colour correction. In addition, we consider two scenarios of powering the X-ray corona, either via accretion, or external to the accretion disc. We use KYNXiltr to fit the observed time lags obtained from intense monitoring of four local Seyfert galaxies (NGC 5548, NGC 4395, Mrk 817, and Fairall 9). We consider various combinations of black hole spin, colour correction, corona height, and fraction of accretion power transferred to the corona. The model fits well the overall time-lags spectrum in these sources (for a large parameter space). For NGC 4593 only, we detect a significant excess of delays in the U-band. The contribution of the diffuse BLR emission in the time-lags spectrum of this source is significant. It is possible to reduce the large best-fitting parameter space by combining the results with additional information, such as the observed Eddington ratio and average X-ray luminosity. We also provide an update to the analytic expression provided by Kammoun et al., for an X-ray source that is not powered by the accretion process, which can be used for any value of colour correction, and for two values of the black hole spin (0 and 0.998).