Relativistic reverberation in the accretion flow of a tidal disruption event

TL;DR

This study detects relativistic reverberation in a tidal disruption event, revealing the inner accretion flow around a supermassive black hole and providing insights into black hole spin and accretion physics.

Contribution

First observation of reverberation from the inner accretion flow in a tidal disruption event, offering new methods to study black hole properties in dormant populations.

Findings

Black hole mass estimated at a few million solar masses

Accretion rate exceeds 100 times the Eddington limit

Reverberation signals originate from the inner accretion flow, not jets

Abstract

Our current understanding of the curved space-time around supermassive black holes is based on actively accreting black holes, which make up only ten per cent or less of the overall population. X-ray observations of that small fraction reveal strong gravitational redshifts that indicate that many of these black holes are rapidly rotating; however, selection biases suggest that these results are not necessarily reflective of the majority of black holes in the Universe. Tidal disruption events, where a star orbiting an otherwise dormant black hole gets tidally shredded and accreted onto the black hole, can provide a short, unbiased glimpse at the space-time around the other ninety per cent of black holes. Observations of tidal disruptions have hitherto revealed the formation of an accretion disk and the onset of an accretion-powered jet, but have failed to reveal emission from the inner accretion flow, which enables the measurement of black hole spin. Here we report observations of reverberation arising from gravitationally redshifted iron K photons reflected off the inner accretion flow in the tidal disruption event Swift J1644+57. From the reverberation timescale, we estimate the mass of the black hole to be a few million solar masses, suggesting an accretion rate of 100 times the Eddington limit or more. The detection of reverberation from the relativistic depths of this rare super-Eddington event demonstrates that the X-rays do not arise from the relativistically moving regions of a jet, as previously thought.