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Gamma-ray discovery sheds light on gravitational waves, black holes

An unexpected light pulse emanating from deep in our universe at the same location where the recently discovered gravitational waves originated raises new questions about the nature of black holes.

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    This visualization shows gravitational waves emitted by two black holes of nearly equal mass as they spiral together and merge. Orange ripples represent distortions of space-time caused by the rapidly orbiting masses. These distortions spread out and weaken, ultimately becoming gravitational waves (purple). Black spheres represent the black hole event horizons, surfaces beyond which nothing can escape. The merger timescale depends on the masses of the black holes. For a system containing black holes with about 30 times the sun’s mass, similar to the one detected by LIGO in 2015, the orbital period at the start of the movie is just 65 milliseconds, with the black holes moving at about one-tenth the speed of light. Space-time distortions radiate away orbital energy and cause the binary to contract quickly. As the two black holes near each other, a new horizon forms around them, creating a single merged black hole that quickly settles into its "ringdown" phase and emits its final gravitational waves. For the 2015 LIGO detection, these events played out in little more than a quarter of a second. This simulation was performed on the Pleiades supercomputer at NASA's Ames Research Center.

    Credit: NASA/J. Bernard Kelly (NASA Goddard), Chris Henze (NASA Ames), Tim Sandstrom (CSC Government Solutions LLC)
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When a team of international scientists announced in February that, after many years of trying, they had finally detected spacetime distortions known as gravitational waves emanating from the collision of two black holes some 1.3 billion light years from Earth, they predicted that this discovery would help researchers learn more about the formation of the universe.

Already, that prediction is coming true.

Scientists from the United States, Italy, Ireland, and Germany who track bursts of gamma rays produced by the most violent events in the universe, such as the collapse of a star, said on Monday that their space telescope has detected tiny light bursts from what appears to be the same event that triggered the recently discovered gravitational waves. Albert Einstein predicted gravitational waves in his general theory of relativity a century ago, and scientists have been working to detect them for 50 years.

Though the power output produced by the brief collision event was 50 times greater than that of all the other stars in the universe combined, scientists – until now – did not expect that it would produce any traceable light, just ripples of gravity that, like ripples formed when a pebble is dropped into a pond, move outward from the collision.

"This is a tantalizing discovery with a low chance of being a false alarm," said Valerie Connaughton, the lead researcher on the gamma-ray burst project from the National Space, Science and Technology Center in Huntsville, Ala., in a NASA announcement.

"But before we can start rewriting the textbooks, we'll need to see more bursts associated with gravitational waves from black hole mergers," said Dr. Connaughton, who is the lead author on a paper on the subject that is now under review by The Astrophysical Journal (but whose results are now publicly available).

If the gamma rays did indeed emanate from the collision that produced the gravitational waves – a hypothesis that will be supported if more more gravity waves and corresponding gamma ray bursts are detected in the future – the discovery could help scientists understand the nature of the formation and collisions of some of the least massive and most poorly understood black holes in our universe.

These so-called stellar-mass black holes are at least a couple of times the mass of the sun and could be located in globular clusters, which are densely packed spherical clusters of stars, or in regions where stars form.

Supermassive black holes, which are located at the center of galaxies and have a mass of billions of suns, tend to be surrounded by dust and gas, allowing them to produce a traceable spark of light when they collide. Stellar-mass black holes, by contrast, are not expected to be surrounded by dust and gas.  

When a collaboration of more than 1,000 scientists from around the globe, who are affiliated with the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), announced in February that they had detected gravitational waves using remarkably sensitive laser interferometers on Earth, they knew based on Einsten's predictions and newer computer simulations that those had rippled from the merging of stellar-mass black holes. This was the first direct observation of these black holes.

"It's the first time the universe has spoken to us through gravitational waves," said David Reitze, executive director of the LIGO experiment, at the time of the announcement.

Less than half a second after LIGO detected gravitational waves, a gamma-ray burst monitor traveling in low-Earth orbit picked up a faint pulse of light lasting only about a second and seemingly from the same source. The team monitoring the bursts calculated a less than 0.2 percent chance that the signal was random.

Judy Racusin, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., said at a press conference that the burst detection team is "cautiously saying [the gamma-ray signal] is potentially associated with the black hole merger" detected by LIGO, as Space.com reported.

Future experiments from LIGO and from the gamma-ray burst monitor will be needed, though the latest findings have already generated a handful of papers from theorists who have put forward models explaining how electromagnetic radiation could come from these less massive, merging black holes.

"There [are] a lot of interesting ideas out there, and it was amazing how quickly those ideas were thrown together," Dr. Racusin said, according to Space.com.

"I think it's great how much theoretical speculation this has caused, and we'll see, maybe, in the future [if any of them pan out] with better observations," she said. 

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