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Published November 2020 | Published + Accepted Version
Journal Article Open

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


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 Kα line is enhanced relative to the continuum by 25 per cent, and the Compton hump by up to a factor of 3. Additional light traveltime 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 3.5–4.5 keV energy range that will be detectable with forthcoming X-ray observatories, representing a unique test of General Relativity in the strong field limit.

Additional Information

© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2020 August 18. Received 2020 August 12; in original form 2020 April 29. DRW was supported by NASA for this work, through Einstein Postdoctoral Fellowship grant number PF6-170160, awarded by the Chandra X-ray Center, operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. JAG acknowledges support from NASA grant 80NSSC17K0345, from the Alexander von Humboldt Foundation, and from the International Space Science Institute (ISSI), Bern, Switzerland, as a member of the International Teams 458 and 486. Computing for this project was performed on the Sherlock cluster. DRW thanks Stanford University and the Stanford Research Computing Center for providing computational resources and support. We thank the anonymous referee for their feedback on the original version of this manuscript. DATA AVAILABILITY. Simulation codes and resulting data sets are available on request to the corresponding author. X-ray reverberation measurements are published in De Marco et al. (2013) and Kara et al. (2016) and the raw data may be obtained from the XMM–Newton Science Archive (http://nxsa.esac.esa.int/nxsa-web).

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Published - staa2566.pdf

Accepted Version - 2008.08083.pdf


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