When Black Holes Feast on Screaming Stars
We don’t normally think of black holes as erupting — they have more of a reputation for gobbling up anything that crosses the event horizon in the popular imagination. But they do give off gamma-ray bursts sometimes.
And those in the middle-weight range, between 100 and 100,000 solar masses, also occasionally exhibit distinctive x-ray emissions when they feed on ill-fated stars. Think of it as a star’s final scream, ripping across light-years of spacetime at a very distinct frequency, akin to a very low D-sharp.
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It’s called a quasi-periodic oscillation (QPO), and it’s the result of matter from the dying star forming an accretion disk as it torn apart, just outside the black hole’s event horizon.”The disk gets heated up and we can see emissions from the disk very close to the black hole in x-rays,” University of Michigan astronomer Rubens Reis explained via press release. “As this matter is falling in, it gives a quasiperiodic wobble.”
Such a signal has only been detected around one supermassive black hole — the type with solar masses ranging in the millions — lurking at the center of a nearby galaxy known as REJ 1034+396, some 576 million light years away.
In a paper published last month in Science, Reis and his fellow astronomers announced that they have detected a QPO signal from a supermassive black hole lurking in the Draco constellation, a whopping 3.9 billion light years away.
That distance means the signal from this x-ray source — dubbed Swift J1644+57 — gives astronomers a chance to learn more about black holes and general relativity at a much earlier era in our universe’s history, when conditions were far more extreme.
NASA’s Swift Gamma-Ray Burst Telescope first spotted the signal last year, and initially astronomers thought it was a gamma-ray burst, until the signal faded away. They concluded that it was the result of a black hole waking up after a quiet phase when a star wandered a bit too close and got gobbled up.
It took subsequent analysis of data collected by two orbiting observatories (first with Suzaku and then with XMM Newton) before Reis and his colleagues were able to detect the telltale oscillation demonstrating that it was, indeed, a QPO.
They were helped by the fact that in addition to the x-rays, the rare event also produced jets that blasted matter outward at very high speeds — and one of those jets just happened to be pointing Earthward. This boosted the QPO signal sufficiently for astronomers to pick it out of all the noise in the data.
So now Reis et al have access to information from the edge of a supermassive black hole, in a galaxy far, far, away — and they fully expect to learn a great deal about the physics under such extreme conditions. Will the principles of general relativity hold firm? We’ll have to wait and see.
Images: (top) Swift J1644+57. Credit: NASA’s Goddard Space Flight Center. (bottom) Swift Gamma-Ray Burst Telescope. Credit: NASA. Both public domain.