Supermassive black hole devours star and becomes X-ray flashlight

The sleeping giant wakes up hungry

Black hole - spaghetti visualisation. Artist's impression.  NASA/JPL-Caltech, CC BY-SA

Astronomers have identified a sleeping black hole that sprung back to life – after trapping a nearby star to be later consumed – due to the curved space-time around it firing X-rays into space, according to research published today in Nature.

Black holes have a strong gravitational pull on any nearby objects, and if anything ventures past its event horizon – the point of no return – it will be ripped apart and swallowed.

Roughly 90 per cent of black holes are dormant, are not actively devouring matter, and do not release any radiation. When a neighbouring star strays too close to a black hole, a dormant black hole will awaken and start a feeding frenzy, also known as a tidal disruption event, releasing jets of radiation during the process.

Astronomers from the University of Maryland and the University of Michigan are the first to study the beams of X-rays released by tidal disruption events around a once-dormant supermassive black hole as a star is trapped in its event horizon.

"Most tidal disruption events don't emit much in the high-energy X-ray band. But there have been at least three known events that have, and this is the first and only such event that has been caught at its peak," said Erin Kara, lead author of the study and a Hubble Postdoctoral Fellow in astronomy at University of Maryland.

The black hole is known as Swift J1644+57 and was found by NASA's Swift satellite, while the large radiation output alerted the attention of other spacecrafts, including NASA's Swift and Suzaku satellites, the European Space Agency's XMM-Newton satellite, and others from the Japan Aerospace Exploration Agency.

The new finding has challenged the belief that radiation emitted from black holes comes from the outer region on its accretion disc – a disc of material surrounding the black hole.

"Before this result, there was no clear evidence that we were seeing into the innermost regions of the accretion disk," Kara said.

"We thought the emission was from the jet pointed at us, or further away and not close to the central black hole. This new study shows us that, actually, we can see this reverberation at work very close to the central black hole."

The X-ray signals discovered by the researchers were found bouncing off the walls of the inner accretion disc. Using a method called X-ray reverberation, the astronomers could map the structure of the accretion disc by measuring the delay in the arrival time between reflected X-ray beams.

Dormant black holes are hard to detect, so finding one that has awoken is valuable.

Chris Reynolds, co-author of the study and a professor of astronomy at University of Maryland, said: "If we only look at active black holes, we might be getting a strongly biased sample. It could be that these black holes all fit within some narrow range of spins and masses. So it's important to study the entire population to make sure we're not biased."

The researchers hope to find out how the radiation beams can be studied to give information about other properties of the black hole such as spin, and how supermassive black holes – millions of times larger than the mass of the sun – grow over time. ®

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