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

TL;DR

This paper demonstrates that short X-ray lags and broad iron line features in active galactic nuclei can be explained by scattering in outflowing winds at large distances, challenging the idea of very compact sources near black holes.

Contribution

It introduces a Monte-Carlo simulation showing how disc winds at large radii can produce observed reverberation lags and line broadening, offering an alternative to near-horizon models.

Findings

Short lags (~2 Rg/c) can arise from distant scattering material.

Outflowing winds at 0.1c can produce broad iron line features.

Line profile shifts depend on variability frequency.

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 $\sim R_g/c$, where $R_g$ 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 $100\,R_g$ from the central source. This gives lags of $\sim2\,R_g/c$, 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 $\sim0.1c$. 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.