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Most black holes that have been observed by our instruments are either stellar size, from a few times to over 100 times the mass of the Sun, or they are supermassive, millions if not billions of times our Sun. Intermediate-mass black holes have been more elusive. Astronomers now believe to have found a new one belonging to this class. And it's all thanks to a cataclysmic explosion: a gamma-ray burst.
Gamma-ray bursts are incredible releases of energy often from extremely bright supernovae or merging stars. The light from one of these events happened to be aligned with a black hole. Due to its huge mass, the black hole warps space-time around it. This warp creates a gravitational lens, whereby the light of the gamma-ray burst gets magnified and distorted by the presence of this black hole.
This allowed researchers to make some estimates about the nature of this object. As reported in Nature Astronomy, the team estimates that the black hole is about 55,000 times the mass of the Sun. The precise mass depends on how far it was located and how far the gamma-ray burst was located, something not know with precision.
Due to the serendipitous alignment, the team was also able to estimate how many of these intermediate-mass black holes should exist. They estimate that there are few thousands of these celestial bodies in a cubic megaparsec (with 1 megaparsec being a million parsec). This tells us that in the neighborhood of the Milky Way, there could be over 40,000 of these objects.
"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," co-author and gravitational lensing pioneer Professor Rachel Webster from the University of Melbourne, said in a statement.
Finding intermediate-mass black hole candidates is important to understand this very mysterious population of objects. But there is also another important aspect that matters here. Researchers believe that these black holes were the seed that grew to be the supermassive black holes we find at the center of galaxies.
"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," explained lead author James Paynter, a graduate researcher from the University of Melbourne.
If the gravitational lens was not caused by an intermediate-mass black hole, it might have been caused by a globular cluster, a spherical collection of stars orbiting a galaxy. However, the team believes that the globular cluster explanation is unlikely though and that the black hole scenario is the more robust scenario out of the two explanations.
