The sleeping giant black hole that awoke to destroy a star


Astronomers used X-rays to discover how the debris in an accretion disc flows towards the centre. Bill Condie reports.


An artist's impression of the thick accretion disk that has formed around a supermassive black hole following the tidal disruption of a star that wandered too close. Flashes of X-ray light near the centre of the disk result in light echoes that allow astronomers to map the structure of the funnel-like flow.
NASA/Swift/Aurore Simonnet, Sonoma State University

Astronomers have for the first time mapped the flow of gas near a monster black hole that awoke from its dormant state to shred a star that passed too close.

The cosmic destruction happened 3.9 billion years ago, although the X-rays produced during it only reached Earth in 2011 when they were first detected by NASA's Swift satellite.

The X-rays, produced by flares near the million-solar-mass black hole known as Swift J1644+57, bounced off the nascent accretion disk and revealed its structure.

The X-rays excite iron ions in the whirling gas near the black hole, causing them to fluoresce.

As an X-ray flare brightens and fades, the gas follows in turn after a brief delay depending on its distance from the source.

“While we don't yet understand what causes X-ray flares near the black hole, we know that when one occurs we can detect its echo a couple of minutes later, once the light has reached and illuminated parts of the flow,” says Erin Kara, from University of Maryland, lead author of the study.

“This technique, called X-ray reverberation mapping, has been previously used to explore stable discs around black holes, but this is the first time we've applied it to a newly formed disc produced by a tidal disruption.”

The disc is made up of stellar debris falling toward a black hole.

Images from Swift's Ultraviolet/Optical (white, purple) and X-Ray telescopes (yellow and red) combined in this composite of Swift J1644+57. The event is seen only in the X-ray image, which is a 3.4-hour exposure taken on 28 March 2011.
NASA/Swift/Stefan Immler

Gas is compressed and heated to millions of degrees before it eventually spills over the black hole’s event horizon, the point beyond which nothing can escape and astronomers cannot observe.

The Swift J1644+57 accretion disk was thicker, more turbulent and more chaotic than stable discs, the study, published in the journal Nature, found.

The high-energy X-rays are, surprisingly, emanating from the inner part of the disc, rather, as scientists expected from a narrow jet of particles.

“We do see a jet from Swift J1644, but the X-rays are coming from a compact region near the black hole at the base of a steep funnel of inflowing gas we're looking down into,” said co-author Lixin Dai, also from the University of Maryland.

“The gas producing the echoes is itself flowing outward along the surface of the funnel at speeds up to half the speed of light.”

Swift J1644+57 is one of only three tidal disruptions that have produced high-energy X-rays, and to date, it remains the only event caught at the peak of this emission.

Astronomers think that some 90% of black holes are dormant and dark, and these episodes briefly activate ones they wouldn’t otherwise know about.

The researchers estimate the mass of the Swift J1644+57 black hole at about a million times that of the sun but did not measure its spin.


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