Returning radiation in strong gravity around black holes: Reverberation from the accretion disc

TL;DR

This paper investigates how strong gravity around black holes causes reflected X-ray emission to return to the accretion disc, significantly affecting observed spectral features and reverberation lags, which are crucial for understanding black hole environments.

Contribution

It provides the first detailed modeling of returning X-ray radiation in strong gravity, quantifying its effects on spectral features and reverberation lags around spinning black holes.

Findings

Up to 47% of reflected X-rays are returned to the disc due to light bending.

Iron K line is enhanced by 25%, and the Compton hump by up to three times.

Reverberation time lags are increased by 25-60% due to returning radiation.

Abstract

We study reflected X-ray emission that returns to the accretion disc in the strong gravitational fields around black holes using General Relativistic ray tracing and radiative transfer calculations. Reflected X-rays that are produced when the inner regions of the disc are illuminated by the corona are subject to strong gravitational light bending, causing up to 47 per cent of the reflected emission to be returned to the disc around a rapidly spinning black hole, depending upon the scale height of the corona. The iron K line is enhanced relative to the continuum by 25 per cent, and the Compton hump by up to a factor of three. Additional light travel time between primary and secondary reflections increases the reverberation time lag measured in the iron K band by 49 per cent, while the soft X-ray lag is increased by 25 per cent and the Compton hump response time is increased by 60 per cent. Measured samples of X-ray reverberation lags are shown to be consistent with X-rays returning to the accretion disc in strong gravity. Understanding the effects of returning radiation is important in interpreting reverberation observations to probe black holes. Reflected X-rays returning to the disc can be uniquely identified by blueshifted returning iron K line photons that are Compton scattered from the inner disc, producing excess, delayed emission in the 3.5-4.5keV energy range that will be detectable with forthcoming X-ray observatories, representing a unique test of General Relativity in the strong field limit.