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

Mizumoto, Misaki and Done, Chris and Hagino, Kouichi and Ebisawa, Ken and Tsujimoto, Masahiro and Odaka, Hirokazu (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). pp. 971-982.


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.

Item Type:Article
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:27 April 2018
Date deposited:10 May 2018
Date of first online publication:02 May 2018
Date first made open access:No date available

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