When Black Holes Collide

Scientists believe nearly every galaxy, ours included, could have, at its center, a supermassive black hole. These black holes can be billions of times the mass of our sun, with a gravitational force so strong not even light can escape beyond the event horizon, making them impossible to see. They are detected by studying the motion of nearby stars, gas and matter accumulated from their surroundings.

The Scientific American reports that in February, a group of scientists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the detection of gravitational waves. This is the first time such waves have been witnessed after their prediction by Albert Einstein 100 years ago. The waves were the result of two black holes, 29 and 36 times the mass of the sun, merging together. As black holes don’t emit light, no light was expected from this merging, as the black holes involved circle each other for millions of years, devouring any matter that may radiate light. Regardless of this prediction, research was conducted on data collected by various telescopes to determine whether there were any light emissions from the region of the two black holes.

“We weren’t actually expecting to see anything,” said Fermi team member Adam Goldstein, a gamma-ray astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

Contrary to expectations, 0.4 seconds after the merge, a faint signal coming from the same region of the sky was detected. Judy Racusin, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said that the Fermi team is “cautiously saying the gamma-ray signal is potentially associated with the black hole merger.”

For a number of reasons, researchers are being cautious in the conclusions they draw from their observations. The signal came from an area of sky below the Fermi satellite, essentially meaning it was caught in the satellite’s peripheral vision and is very weak. It is, according to Goldstein, uncertain whether the signal was, in fact, weak or appeared so due to its position relating to the spacecraft’s detectors. Based on what the Fermi team knows about the gamma-ray sky, Goldstein said there was about a “one in 500 chance” that the signal appeared in roughly the same area at the same time. The region of the black hole collision could be determined to be in an area spanning 600 square degrees of the sky, with Fermi’s detection narrowing it down to about a third of that. This still leaves a wide area from which the signals could have come.

A paper released by the INTErnational Gamma-Ray Astrophysics Laboratory(INTEGRAL), showed that it did not detect the signal picked up by Fermi, although the INTEGRAL’s instrumentation was not designed as a gamma ray detector, making it impossible to compare what the two see.

The new paper about Fermi’s gamma-ray detection is currently undergoing peer preview by the Astrophysical Journal.

Goldstein said, “It is important to note we would not have reported this event just by itself, the reason we are publishing is because our data is public, obviously, and the most appropriate people to do this analysis [are in] the instrument team. And so there was particular pressure on us putting out a paper for this.”

A number of papers have been posted on the open-access science paper website arXiv.org denoting scenarios in which colliding black holes may result in such a gamma ray burst.

“There [are] a lot of interesting ideas out there, and it was amazing how quickly those ideas were thrown together. The main idea is that you need to get some sort of matter outside those black holes, some sort of gas to accelerate…. 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,” Racusin said.

With as many as 100 such black hole merges per year expected, Goldstein said that plenty of opportunities will arise to see if other merges will have similar gamma ray signals.

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