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LIGO and NASA may have observed a gamma-ray burst from colliding black holes

LIGO researchers made history in February with their first-ever direct detection of gravitational waves, confirming predictions made by Albert Einstein a century ago. The waves they detected came from the collision of two black holes, each about thirty solar masses, which had finally spiraled close enough to merge and leave the telltale waves. Just to produce the gravitational waves LIGO observed, the cataclysm liberated the mass energy of three of our suns — in less than a quarter of a second.

After the collision, researchers pored over the data archives of many telescopes, looking for any signs of light emissions from the origin of the gravitational waves. “We weren’t actually expecting to see anything,” said gamma-ray astrophysicist Adam Goldstein, a member of the Fermi team. Why? Merging black holes take millions of years to collide. Scientists had generally expected them to sweep out their neighborhood of any gas or other matter, long before they got close enough to collide. While matter falling into black holes can release light, the black hole itself doesn’t. Furthermore, when black holes collide, they’re expected to merge “cleanly,” without producing light. But the scientists did see something. In the same place LIGO was looking when it saw the gravitational waves, NASA’s Fermi telescope had seen a gamma-ray burst, less than a second after the black holes physically merged.

Chances are good that the gamma-ray burst did come from the black-hole merger, but more evidence is necessary before anyone can make definitive claims. Based on what the Fermi team knows, Goldstein said, there’s about a 1 in 500 chance that the gamma-ray signal is not associated with the gravitational wave signal. “This is a tantalizing discovery with a low chance of being a false alarm,” said Valerie Connaughton, lead author of the Fermi collision report and GBM team member at the National Space, Science and Technology Center in Huntsville, Alabama. “But before we can start rewriting the textbooks, we’ll need to see more bursts associated with gravitational waves from black hole mergers.”

Goldstein and Judy Racusin, of NASA’s Goddard Space Flight Center, discussed the evidence for and against their conclusions at the April meeting of the American Physical Society this week. In particular, there was a paper released by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), which showed that their instrument didn’t see what the Fermi telescope did. But the INTEGRAL instrument wasn’t primarily a gamma-ray burst detector, Goldstein said, so it’s “actually not very clear if [INTEGRAL] would have seen it or not.” Goldstein and Racusin expect that LIGO will have a 90% probability of finding more black hole mergers in the near future — so we’ll have plenty of opportunities in the next few years to see whether we can make this observation on other binary black holes.

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