Astronomers have already tracked dozens of black hole mergers via gravitational waves, but now they are reporting a very special case. On May 21, 2019, the detectors of the LIGO and Virgo observatories registered a signal that came from an extremely unusual collision. With 85 and 66 solar masses, the two original black holes were already far heavier than all previously observed. But the object resulting from their merging is both a record and a mystery at 142 solar masses. Because with this mass it could be an intermediate black hole – a class of black holes that has never been clearly proven.
Almost all known black holes fall into two categories: Either they are super-massive giants of hundreds of thousands to ten billion solar masses and are located in the center of galaxies. Or they are stellar black holes that were created by the supernova of a massive star. According to current theory, these black holes cannot contain more than about 65 solar masses. This is because even heavier starting stars do not explode as a supernova, but go through short unstable episodes in which they each repel a significant proportion of their mass. As a result, these stars “shrink” so much that when they explode, only a black hole with a maximum of 65 solar masses is created. At the other end of the spectrum are stars with more than 200 solar masses that neither shrink nor explode, but instead collapse directly to form a black hole – this creates black holes from around 120 solar masses. That means: In the range of 65 to 120 solar masses there is a gap in which there should be no black holes – astrophysicists refer to this as a pair instability gap.
Extremely heavy and quite loud
But now researchers from the LIGO and Virgo collaboration are reporting a gravitational wave signal that testifies to the existence of two black holes in this gap. The signal, named GW190521, was detected on May 21, 2019 both by the two LIGO detectors in the USA and by the Virgo gravitational wave detector in Italy. The tiny vibrations of space-time lasted only about a tenth of a second and comprised four oscillation cycles – this corresponds to the last two orbits of two black holes before they merged. What was unusual, however, was the high intensity and low frequency of the vibrations, which indicated the merging of two particularly massive objects. “The signal was less of a chirp, as we typically detect it, than a real ‘bang’ – it is the most massive signal that LIGO and Virgo have ever observed,” says Nelson Christensen of the French national research center CNRS.
From the signal characteristics, the researchers conclude that the gravitational waves originate from the merging of two particularly massive black holes. “The larger one weighed around 85 solar masses, the smaller one was closer to 66 solar masses,” reports the LIGO collaboration. “Both black holes are therefore far more massive than all previously detected by Virgo and LIGO.” The black hole resulting from this merger was correspondingly heavy: the researchers estimate it to be around 142 solar masses. Thus GW190521 is the amalgamation with the highest total mass ever observed. The release of gravitational waves during this event was correspondingly large. According to the physicists, the energy equivalent of seven times the solar mass was emitted during this collision. This made the gravitational waves strong enough to reach us even from the enormous distance of more than seven billion light years. The merger of the two black holes took place about seven billion years ago – then the universe was half as old as it is today.
Intermediate black hole and a precursor to the mass gap
With the unusually large mass of its black holes, the gravitational wave event GW190521 causes the physicists to need explanations. “This event raises more questions than it answers,” says LIGO member Alan Weinstein of the California Institute of Technology. “From the point of view of physics, that’s a very exciting thing.” One factor is the black hole that was created during this merger: With 142 solar masses, it moves in an area where the so-called intermediate black holes could be. These are 100 to 100,000 times heavier than the sun and are therefore in the intermediate area between the stellar black holes and the supermassive black holes of the galaxy nuclei. Astrophysicists have long suspected that these intermediate forms must exist in the cosmos, but so far there have only been indirect indications of their existence – there was no clear evidence. “Now we have evidence that these intermediate black holes exist,” says Christopher Berry of Northwestern University in Evanston.
(Video: Max Planck Institute for Gravitational Physics)
Even more surprising and puzzling is the heavier of the two precursors of this intermediate black hole. Because with 85 solar masses it lies exactly in the pair instability gap of the stellar black holes. “The fact that we see a black hole in the middle of this mass gap will make a lot of astrophysicists wonder how such a black hole came about,” says Christensen. He and his colleagues suspect that this black hole may itself be the product of a previous merger. Then GW190521 would be an example of the hierarchical mergers that have so far only been postulated theoretically – collisions of two black holes, which themselves have also emerged from mergers. Such series of increasingly massive mergers could occur where many stars close to one another reach the end of their life cycle and become black holes via supernovae – for example in star clusters or in the dense centers of galaxies.
It is not yet clear how the gravitational wave event GW19052 came about and how its actors came about. The question of whether black holes of this mass are cosmic outliers or only represent the difficult end of the previously known mass spectrum has not yet been clarified. “Hopefully, when we have analyzed all of the merging of black holes that LIGO and Virgo observed in their third observation run, we will know more,” says Karsten Danzmann from the Max Planck Institute for Gravitational Physics.
Source: LIGO Collaboration and Virgo Collaboration Physical Review Letters, Astrophysical Journal Letters