X-ray short-time lags in the Fe-K energy band produced by scattering clouds in active galactic nuclei

• Published in: Monthly notices of the Royal Astronomical Society Vol. 478; no. 1; pp. 971 - 982
• Main Authors: Mizumoto, Misaki, Done, Chris, Hagino, Kouichi, Ebisawa, Ken, Tsujimoto, Masahiro, Odaka, Hirokazu
• Format: Journal Article
• Language: English
• Published: Oxford University Press 21.07.2018
• Subjects: galaxies: Seyfert; black hole physics; galaxies: active; X-rays: galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sty1114

Abstract X-rays illuminating the accretion disc in active galactic nuclei give rise to an iron K line and its associated reflection spectrum that are lagged behind the continuum variability by the light-travel time from the source to the disc. The measured lag time-scales in the iron band can be as short as ∼Rg/c, where Rg is the gravitational radius, which is often interpreted as evidence for a very small continuum source close to the event horizon of a rapidly spinning black hole. However, the short lags can also be produced by reflection from more distant material, because the primary photons with no time-delay dilute the time-lags caused by the reprocessed photons. We perform a Monte Carlo simulation to calculate the dilution effect in the X-ray reverberation lags from a half-shell of neutral material placed at 100 Rg from the central source. This gives lags of ∼2 Rg/c, but the iron line is a distinctly narrow feature in the lag-energy plot, whereas the data often show a broader line. We show that both the short lag and the line broadening can be reproduced, if the scattering material is outflowing at ∼0.1c. The velocity structure in the wind can also give shifts in the line profile in the lag-energy plot calculated at different frequencies. Hence we propose that the observed broad iron reverberation lags and shifts in profile as a function of frequency of variability can arise from a disc wind at fairly large distances from the X-ray source.