Scientists watch ‘dormant’ black hole consume a star

For the first time, astronomers have conducted detailed observations of a tidal disruption event involving a once-dormant supermassive black hole – a breakthrough which could soon make it possible to reliably measure the spin of these gravitationally-dense regions of spacetime.

Nearly 90 percent of the largest black holes in the universe are dormant, meaning that they are not actively consuming matter or giving off radiation. In some cases, however, a star can wander too close to these sleeping giants and whet their appetites, so to speak, causing a tidal disruption event – a phenomenon in which the star is yanked apart by the black hole’s tidal forces.

Now, a team of astronomers from the University of Michigan and the University of Maryland have become the first scientists to successfully document X-rays in the depths of a previously- dormant black hole’s newly-acquired accretion disk (the large cloud of star remains encircling the black hole) following such an event.

Not only does this mark the first time that such observations have been made for a dormant supermassive black hole, but the researchers explained in a statement that the techniques they used could ultimately be used to accurately measure black hole spin. Their findings have been published in Wednesday’s advanced online edition of the journal Nature.

Astronomers surprised at the location of black hole X-rays

As part of their research, Erin Kara, a Hubble Postdoctoral Fellow in astronomy at UMD and the Joint Space-Science Institute, and her colleagues were able to determine the shape and activity of the accretion disk near Swift J1644+57, a tidal disruption which took place in the center of a tiny galaxy located approximately 3.8 billion light years away in the Draco constellation.

“Most tidal disruption events don’t emit much in the high-energy X-ray band,” Kara explained in a statement. “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.”

“NASA’s Swift satellite saw it first and triggered the European Space Agency’s XMM-Newton satellite and the Japanese Aerospace Exploration Agency and NASA’s Suzaku satellite to target it for follow-up. So we have excellent data,” she added. “We’re lucky that the one event we have is showing us all these exciting new things.”

Kara noted that she was surprised that X-rays would be able to originate in the depths of a tidal disruption’s accretion disk, as astronomers had long believed that during such an event, energetic X-rays would be created further away from the black hole in its relativistic jets (large beams of particles that are ejected by the black hole and which reach close to the speed of light).

“Before this result, there was no clear evidence that we were seeing into the innermost regions of the accretion disk. We thought the emission was from the jet pointed at us, or further away and not close to the central black hole,” she explained. “This new study shows us that, actually, we can see this reverberation at work very close to the central black hole.”

How this breakthrough could help measure black hole spin

Previously, the majority of our knowledge about supermassive black holes had come from a relatively small percentage of objects that are actively accumulating and consuming matter, the study authors explained. However, research suggests that such black holes only make up about one-tenth of the total population, making the new data extremely significant.

“Understanding the black hole population in general is important,” said study co-author Chris Reynolds, a professor of astronomy at UMD. “Black holes have played an important role in how galaxies evolved. So even if they’re dormant now, they weren’t before. 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.”

Because the Swift J1644+57 event resulted in the uber-quick consumption of the shredded star, it exceeded the Eddington Limit (the theoretical maximum for how quickly a black hole is able to consume matter) for a brief period of time – a discovery which could help experts determine how supermassive black holes are able to grow to sizes several million times that of our sun.

Kara and her colleagues used a technique known as X-ray reverberation mapping to outline the inside of the accretion disk. This method is similar to the way in which scientists map seafloors using the intervals between echoes, except that instead of sound, they detected delays between the arrivals of X-ray signals reflected from iron atoms in the accretion disk. While they are not yet able to use this technique to measure the speed and direction of a black hole’s spin, they are anticipating being able to do so in the near future.

“Looking at tidal disruption events with reverberation mapping might help us probe the spin of black holes in the future,” said Reynolds. “But just as importantly, we can follow along after an event and watch how the accretion disk spins down and energy dissipates as the black hole returns to a quiescent state. We might finally be able to observe all of these various states that, so far, we only know from theory textbooks.”

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Image credit: NASA/Swift/Aurore Simonnet, Sonoma State U.

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