Highly eccentric black-hole merger is identified in LIGO–Virgo observation

Astronomers in the US have found strong evidence that a merging pair of black holes with highly eccentric orbits has been seen by the LIGO–Virgo gravitational wave detectors. The team, led by Richard O’Shaughnessy at Rochester Institute of Technology, made the discovery after running an extensive series of simulations – which they used to recreate the gravitational waveforms originating from a merger that was spotted in 2019. Their results suggest that the merger was the result of a chance encounter between two black holes in a dense star cluster.

The latest theories of stellar evolution place an upper limit of around 50 solar masses on the sizes of black holes produced through supernovae. However, not all observed black holes appear to obey this rule. In 2019, the LIGO–Virgo observatories detected GW190521: a gravitational-wave signal generated by the most massive pair of merging black holes observed to date, each measuring over 70 solar masses.

To explain why these hefty objects were well above the apparent mass limit, some astronomers say that they could have each been second-generation black holes – which are themselves created by black-hole mergers. Afterwards, the two bodies may have been caught in each other’s gravity by chance to form a binary that merged. Such multiple mergers are likely to occur in regions that are densely populated by black holes, such as galactic nuclei.

Lost eccentricity

So far, most mergers observed by astronomers appear to have involved pairs of black holes in highly circular orbits – whereby both objects orbit in circles around their centre of mass. Such systems would have probably begun as binary stars and would have remained stable for billions of years before merging, so that any orbital eccentricity would be lost through emissions of gravitational waves.

However, binary systems formed in chance encounters would begin in highly eccentric orbits – with both objects in elliptical orbits around their centre of mass. If the orbital radii were small, the black holes would merge before the eccentricity was radiated away.

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