A theoretical study of the time lags due to Comptonization and the constraints on the X-ray corona in AGNs
Zhang, W.; Papadakis, I. E.; Dovčiak, M.; Bursa, M.; Karas, V.
March 2023, Monthly Notices of the Royal Astronomical Society, Volume 519, Issue 4, pp.4951-4965
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 that 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 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.