Collision of the giants was eccentric – First potential evidence of eccentric orbits in merging black holes –

Eccentric giants: The most massive collision between two black holes to date may also have been the first known eccentric collision of such gravitational giants. This is suggested by agreements between the gravitational-wave data and model simulations. If confirmed, this could explain why the black holes were more massive than current theory allows. Because then they could have received mass from outside beforehand.

In May 2019, the LIGO and Virgo observatories’ gravitational-wave detectors caught the signal of a highly unusual event. Because at GW190521 two black holes had apparently merged, which were heavier than they should be with 85 and 66 solar masses. Because according to current theory, there can be no black holes in the range of 65 to 120 solar masses. The reason: Supernovae do not leave such heavy relics behind, and if a star collapses directly into a black hole, at least 120 solar masses remain.

The large mass of collision GW190521 is puzzling, but may result from previous mergers. © LIGO/Caltech/ MIT/R. Hurt (IPAC)

But how is GW190521 then to be explained? So far, astrophysicists can only speculate. Some suspect the effect of cosmic expansion behind this, others consider it likely that such “overweight” black holes grow from several consecutive mergers. Favorable conditions for this can exist in dense star clusters or in the centers of galaxies.

Was the collision eccentric?

V. Gayathri from the University of Florida in Gainesville and her colleagues may have found a possible indication of such a hierarchical merger in GW190521. They specifically looked for evidence that the two black holes orbited each other in eccentric orbits before their collision. Because this is considered an indication that a binary system was exposed to external disturbances, such as those that occur in dense star clusters.

The problem, however, is that it is extremely difficult to prove that the orbits are eccentric using gravitational waves. Because in the signals, which usually last less than a second, no clear wave feature has been found so far that would reveal such an eccentricity. It is therefore not possible to determine the shape of the orbits of both black holes with current analysis methods.

match found

Gayathri and her team therefore took a different approach: They simulated 611 eccentric and 920 non-eccentric mergers of black holes of different masses in the model. They also reconstructed the gravitational waves released in the process and searched for matches with GW190521.

The result: The best match between model and observation was achieved in a scenario in which two black holes orbit each other in significantly eccentric orbits. According to the data, the eccentricity could be around 0.69. For comparison, Earth’s orbit has a value of 0.017 on a scale ranging from zero to one, the extremely eccentric orbit of a recently discovered exoplanet is 0.96.

Potential indication of hierarchical mergers

If confirmed, GW190521 could be the first evidence of a binary black hole merger with eccentric orbits. In addition, Gayathri and her team conclude from their simulations that between eight and 13 percent of the gravitational-wave events detected by LIGO and Virgo could originate from such eccentric collisions.

“This represents a major advance in our understanding of how black holes merge,” says co-author Manuela Campanelli of the Rochester Institute of Technology in New York. At the same time, this could provide an indication that GW190521’s unusually massive black holes are in fact the result of a hierarchical merger. According to the team, this scenario can lead to both high eccentricity and high mass. (Nature Astronomy, 2022; doi: 10.1038/s41550-021-01568-w)

Quelle: Rochester Institute of Technology

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