A theoretical study of the time-lags due to Comptonization and the constraints on the X-ray corona in AGN

TL;DR

This paper models Fourier time-lags caused by Comptonization in AGN X-ray coronae using relativistic Monte Carlo simulations, providing equations to interpret observed time-lags and luminosities, and demonstrating consistency with real galaxy data.

Contribution

It introduces a detailed relativistic Monte Carlo model for Comptonization-induced time-lags in AGN coronae and derives equations to connect physical parameters with observable signals.

Findings

Time-lags remain constant up to a characteristic frequency

Equations relate corona parameters to observed time-lags and luminosity

Model reproduces observed X-ray time-lags in active galaxies

Abstract

We study the Fourier time-lags due to the Comptonization of disc-emitted photons in a spherical, uniform, and stationary X-ray corona, which located on the rotational axis of the black hole. We use Monk, a general relativistic Monte-Carlo radiative transfer code, to calculate Compton scattering of photons emitted by a thin disc with a Novikov-Thorne temperature profile. We find that the model time-lags due to Comptonization remain constant up to a characteristic frequency and then rapidly decrease to zero at higher frequencies. We provide equations which can be used to determine the time-lags and cross spectra for a wide range of values for the corona radius, temperature, optical depth, height, and for various accretion rates and black hole masses. We also provide an equation for the X-ray luminosity of a single corona, as a function of the its characteristics and location above the disc. Remarkably, the observed X-ray time-lags of nearby, bright active galaxies can be successfully reproduced by inverse Comptonization process of multiple dynamic coronae.