Astronomy: Waking sleeping giants

A normally dormant supermassive black hole (SMBH) tearing apart a nearby star is reported in a paper published online in Nature this week. The study analyses X-ray data collected as the star experienced this ‘tidal disruption event’ and accreted onto the SMBH, and opens up a new way of studying gravity effects in normally dormant black holes and, possibly, measuring their spin.

Although our current understanding of space-time around SMBHs is based on actively accreting black holes, 90 per cent of SMBHs are dormant. Tidal disruption events offer an opportunity to study this large population of normally dormant SMBHs.

Erin Kara and colleagues reanalyse previously available X-ray data for a likely tidal disruption event that was detected in 2011, the so-called ‘Swift J1644+57’, using a technique known as X-ray reverberation mapping. They report observations of X-ray echoes, or reverberations, from iron photons. Analysis of these reverberations reveals that they are from the inner part of an accretion flow, with the reflecting gas flowing outwards at up to half the speed of light.

Although the authors do not estimate the spin of the black hole, they propose that, with future improvements in the modelling of such flows, it will be possible to measure black hole spin not only in the 10 per cent of persistently accreting black holes, but also in the 90 per cent of dormant black holes in the Universe.

doi: 10.1038/nature18007

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