X-ray reverberation modelling of the continuum, optical/UV time-lags in quasars

TL;DR

This study demonstrates that X-ray reverberation models can accurately explain continuum time lags in quasars, allowing estimation of black hole and corona parameters, with implications for understanding AGN accretion physics.

Contribution

The paper applies sophisticated X-ray reverberation models to quasar time-lag data, providing new insights into black hole spin, corona height, and disc structure in AGN.

Findings

X-ray reverberation explains observed time lags well.

Corona height is estimated to be >40 gravitational radii.

Model fits are consistent for both non-rotating and maximally rotating black holes.

Abstract

Context: Extensive, multi-wavelength monitoring campaigns of nearby and higher redshift active galactic nuclei (AGN) have shown that the UV/optical variations are well correlated with time delays which increase with increasing wavelength. Such behaviour is expected in the context of the X-ray thermal reverberation of the accretion disc in AGN. Aims: Our main objective is to use time-lag measurements of luminous AGN and fit them with sophisticated X-ray reverberation time-lags models. In this way we can investigate whether X-ray reverberation can indeed explain the observed continuum time lags, and whether time-lag measurements can be used to measure physical parameters such as the X-ray corona height and the spin of the black hole (BH) in these systems. Methods: We use archival time-lag measurements for quasars from different surveys, and we compute their rest frame, mean time-lags spectrum. We fit the data with analytical X-ray reverberation models, using $\chi^2$ statistics, and fitting for both maximal and non spinning BHs, for various colour correction values and X-ray corona heights. Results: We found that X-ray reverberation can explain very well the observed time lags, assuming the measured BH mass, accretion rate and X-ray luminosity of the quasars in the sample. The model agrees well with the data both for non-rotating and maximally rotating BHs, as long as the corona height is larger than $\sim 40$ gravitational radii. This is in agreement with previous results which showed that X-ray reverberation can also explain the disc radius in micro-lensed quasars, for the same corona heights. The corona height we measure depends on the model assumption of a perfectly flat disc. More realistic disc models may result in lower heights for the X-ray corona.