Astronomers Use Gravitational Lensing Effect to Detect Intermediate-Mass Black Hole
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A gravitationally lensed gamma-ray burst, designated GRB 950830, helped astronomers detect a black hole about 55,000 times more massive than the Sun.

An artist’s impression of an intermediate-mass black hole. Image credit: Sci-News.com.

Astronomers know that stellar-mass black holes — compact ranging from about 10 times to 100 times the Sun’s mass — are the remnants of dying stars, and that supermassive black holes — those with masses over 100,000 times the mass of the Sun — inhabit the centers of most galaxies.

But scattered across the Universe are a few apparent black holes of a more mysterious type.

Ranging from 100 to 100,000 solar masses, these intermediate-mass black holes have long been posited to reside in the cores of globular clusters. Yet direct observational signatures of their existence are elusive.

“The newly-discovered black hole could be an ancient relic — a primordial black hole — created in the early Universe before the first stars and galaxies formed,” said Professor Eric Thrane, an astronomer in the School of Physics at Monash University and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

“These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.”

“The discovery sheds new light on how supermassive black holes form,” said James Paynter, a Ph.D. student in the School of Physics at the University of Melbourne.

“While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe.”

The new intermediate-mass black hole was detected thanks to GRB 950830, a gravitationally lensed short burst of gamma-rays.

The gamma-ray burst, emitted by a pair of merging stars, was observed to have a tell-tale ‘echo.’

This echo was caused by the intervening intermediate-mass black hole, which bent the path of the light on its way to Earth, so that astronomers see the same flash twice.

Powerful software developed to detect black holes from gravitational waves was adapted to establish that the two flashes are images of the same object.

“Our findings have the potential to help scientists make even greater strides,” said Professor Rachel Webster, also from the School of Physics at the University of Melbourne.

“Using this new black hole candidate, we can estimate the total number of these objects in the Universe.”

“We predicted that this might be possible 30 years ago, and it is exciting to have discovered a strong example.”

The findings appear in the journal Nature Astronomy.

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J. Paynter et al. Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst. Nat Astron, published online March 29, 2021; doi: 10.1038/s41550-021-01307-1

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