Seven billion years ago, two huge black holes came together to become one. The latter would be of intermediate-mass, an “elusive” class of object.
LIGO (United States) and VIRGO (Italy) are the two largest gravitational wave detectors in the world. In September 2015, these two instruments detected their first ripples in the fabric of space-time: signals generated by the fusion of two black holes at about 1.3 billion light-years. Since then, other unions have been detected. Some have involved other black holes, while others have involved neutron stars.
A team of researchers today announced that they have recorded the most massive black hole collision ever detected using gravitational waves. The two objects, of 66 and 85 solar masses, then formed a single black hole as massive as 142 suns (there was a loss of mass during the fusion).
In addition, these two black holes met about seven billion light-years from Earth. In other words, the waves resulting from their forced union have traveled for half the age of the universe, before finally reaching us.
“Elusive” black holes
Interestingly, this last black hole also falls into the class of intermediate-mass black holes.
There are indeed different types of black holes in the Universe. On the one hand, we have objects of stellar mass (less than 100 solar masses), formed by the collapse of a massive star. And on the other hand the “supermassive”, with masses equivalent to millions or billions of suns. Between these extremes are more discrete members of the black hole family: those of intermediate-mass.
These are particularly difficult to find. To do this, it is necessary to wait until a star passes a little too close to be disturbed. The black hole, once again active, can then emit X-rays betraying their presence in an indirect way. The Hubble Telescope recently had the opportunity to isolate one of these objects. This new detection made by LIGO and VIRGO, therefore, confirms their existence.
Finally, the last point, the heavier of the two original black holes contradicts what we know about stellar evolution. The latter does indeed fall into the so-called “pair-instability” gap.
“Based on our understanding of how stars age and change, we would expect to find black holes with less than 65 solar masses or more than 135 solar masses, but none in between,” says Frank Ohme, of the Max Planck Institute for Gravitational Physics (AEI) in Hanover.
However, the black hole of 85 solar masses involved in this merger fits into this gap. To justify its presence, the researchers suggest it could be the product of previous mergers involving either two small black holes or two larger stars.
Details of this work have been published in Physical Review Letters and The Astrophysical Journal Letters. Note that the two observatories detected 56 possible gravitational wave events during their third operating cycle, between April 1, 2019, and March 27, 2020. Of this sample, 52 others are still being analyzed.
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