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Curious light signal alerts scientists to possible black hole collision

An unusual light signal coming from the centre of a far-off galaxy alerted scientists to a binary black hole system which could be merging into one.

Researchers, using data from NASA’s Galaxy Evolution Explorer (GALEX) and NASA’s Hubble Space Telescope, believe it is the most compelling confirmation yet for the existence of these merging black holes.

The black hole duo, called PG 1302-102, was first identified earlier this year using ground-based telescopes. They are the tightest orbiting pair detected so far, with gravitational system enmeshed in an intricate dance.

The two are separated by space not much bigger than the diameter of our solar system and are expected to collide and merge in less than a million years, triggering a titanic blast with the power of 100 million supernovae.

Researchers are studying this pair to better understand how galaxies and the monstrous black holes at their cores merge – a common occurrence in the early universe. But as common as these events were, they are hard to spot and confirm.

The findings are presented in a study published in Nature.

They tracked the changing light patterns of the black holes over the past 20 years using ultraviolet data from GALEX and Hubble.

“We were lucky to have GALEX data to look through,” said co-author David Schiminovich of Columbia University.

“We went back into the GALEX archives and found that the object just happened to have been observed six times.”

The ultraviolet light helped scientists test a prediction of how the black holes generate a cyclical light pattern.

The idea is that one of the black holes in the pair is giving off more light it is gobbling up more matter than the other one, and this process heats up matter that emits energetic light. As this black hole orbits around its partner every five years, its light changes and appears to brighten as it heads toward us.

“It’s as if a 60-Watt light bulb suddenly appears to be 100 Watts,” says Daniel D’Orazio, lead author of the study from Columbia. 

“As the black hole light speeds away from us, it appears as a dimmer 20-Watt bulb.”

One set of changes has to do with the “blue shifting” effect, in which light is squeezed to shorter wavelengths as it travels toward us.

Another reason is the enormous speed of the black hole, travelling at nearly 7% the speed of light.

Though it takes the black hole five years to orbit its companion, it is traveling vast distances. It would be as if a black hole lapped our entire solar system from the outer fringes, where the Oort cloud of comets lies, in just five years. At speeds as high as this, which are known as relativistic, the light becomes boosted and brighter.

You can find more information about GALEX on the NASA website here.

Bill Condie

Bill Condie

Bill Condie is a science journalist based in Adelaide, Australia.

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