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

Mizumoto, Misaki; Done, Chris; Hagino, Kouichi; Ebisawa, Ken; Tsujimoto, Masahiro; Odaka, Hirokazu

Abstract

X-rays illuminating the accretion disc in active galactic nuclei give rise to an iron K line and its associated reflection spectrum which are lagged behind the continuum variability by the light-travel time from the source to the disc. The measured lag timescales 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.

Citation

Mizumoto, M., Done, C., Hagino, K., Ebisawa, K., Tsujimoto, M., & Odaka, H. (2018). X-ray short-time lags in the Fe-K energy band produced by scattering clouds in active galactic nuclei. Monthly Notices of the Royal Astronomical Society, 478(1), 971-982. https://doi.org/10.1093/mnras/sty1114

• Journal Article Type: Article
• Acceptance Date: Apr 27, 2018
• Online Publication Date: May 2, 2018
• Publication Date: Jul 21, 2018
• Deposit Date: May 10, 2018
• Publicly Available Date: May 10, 2018
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 478
• Issue: 1
• Pages: 971-982
• DOI: https://doi.org/10.1093/mnras/sty1114
• Public URL: https://durham-repository.worktribe.com/output/1359291