Skip to main content
Gravity

Gravity

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

01 Feb 2022
Merging black holes
Chance encounter: Artist’s impression of the gravitational waves emitted by merging black holes. (Courtesy: LIGO/T Pyle)

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.

If this occurred, information about the eccentricity would be imprinted on gravitational waves produced during the merger. However, identifying the signatures of eccentricity in LIGO–Virgo observations has proven to be difficult.

To address this limitation, O’Shaughnessy’s team did simulations of GW190521-like mergers using 611 eccentric, and 920 non-eccentric orbits. These simulations covered a full range of possible eccentricities and were also scaled to correspond to a range of black-hole masses. The result was nearly 100,000 different gravitational-wave signals. The simulations were carried out on local and national supercomputers across the US and took nearly a year to complete.

O’Shaughnessy’s team compared their results with the real waveform of GW190521, to determine which of their simulations offered the best match. For the first time, they showed that a LIGO–Virgo observation is highly consistent with a highly eccentric merger. As the capability of LIGO and Virgo’s detectors continue to rapidly improve, the astronomers now hope that their approach will identify future cases of eccentric mergers and chance black-hole encounters.

The research is described in Nature Astronomy.

Copyright © 2024 by IOP Publishing Ltd and individual contributors