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Gravitational waves: The first ‘mixed’ black hole and neutron star pairs observed

29 June 2021

The Virgo, LIGO and KAGRA scientific collaborations today announced the first observation ever of binary systems consisting of a black hole and a neutron star (NSBH). This was made possible by the detection, in January 2020, of gravitational signals emitted by two systems, in which a black hole and a neutron star, rotating around each other, merged into a single compact object. The existence of these systems was predicted by astronomers several decades ago, but they had never been observed with confidence, either through electromagnetic or gravitational signals, until now. The result and its astrophysical implications have been published today in The Astrophysical Journal Letters. Mario Spera, now in the SISSA scientific staff and member of Padua's INFN section, has taken part in the research.

On the 5th of January, 2020, the Advanced LIGO detector in Livingston, Louisiana in the US, and the Advanced Virgo detector in Italy, observed a gravitational wave produced by the last few decaying orbits, before the merging, of a NSBH pair; just ten days later a second gravitational-wave signal from the inspiral and merger of a similar binary system, was observed, this time by both Advanced LIGO detectors and the Virgo detector. These two events, nicknamed GW200105 and GW200115 (from the dates of their detection), represent the first observations of gravitational waves generated by a mix of neutron stars and black holes.

The gravitational signals detected in January encode valuable information about the physical features of the systems, such as the mass and distance of the two NSBH pairs, as well as about the physical mechanisms that have generated them and bring them to collapse.  The signal analysis has shown that the black hole and neutron star that created GW200105 are, respectively, about 8.9 times and 1.9 times the mass of our Sun (Mo) and their merger happened around 900 million years ago, hundreds of millions of years before the first dinosaurs appeared on Earth. For the GW200115 event, the Virgo and LIGO scientists estimate the two compact objects had masses around 5.7 Mo (BH) and 1.5 Mo (NS) and that they merged almost 1 billion years ago.

"This is the first confirmation of the existence of a new type of astrophysical system: a mixed binary, that is a binary consisting of a black hole and a neutron star. This confirmation comes from gravitational waves” comments Mario Spera, member of the Virgo scientific collaboration and, for about a year, researcher at SISSA. "Despite many theoretical studies had already predicted their existence" Spera says, "we had not been able to get any confirmations of the existence of mixed binaries, so far. After several years, in just 10 days, the Virgo and LIGO interferometers detected GW200105 and GW200115, both identified as neutron star – black hole binaries". 

The astrophysical interpretation of the detected events is still shrouded in mystery since it is not clear what is the formation channel and the evolutionary history of the objects that originated GW200105 and GW200115. Future research will have to investigate these aspects.


About the image: Voxel Burst is an artistic interpretation of a generic Black Hole Neutron Star merger event. Credit: Carl Knox, OzGrav -Swinburne University