Relativistic reverberation in the accretion flow of a tidal disruption
  event
  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.
