Gravitational waves from the merger of a black hole and a neutron star may have been spotted for the first time by the LIGO and Virgo detectors. The signal was detected on 14 August and appears to come from an event that occurred about 900 million light-years away.
The detection has been logged on the Gravitational-Wave Candidate Event Database, where the LIGO–Virgo team say that there is a greater than 99% probability that the gravitational waves are from a black-hole–neutron-star merger. It is likely that the larger object was heavier than five solar masses and the smaller object lighter than three solar masses.
There is no indication so far that any electromagnetic observations of the object have been made by any other telescopes. This will come as a disappointment to the astronomy community because “multimessenger” observations across the electromagnetic spectrum could provide important insights into black holes, neutron stars and how they merge. So far, the only multimessenger observations to involve gravitational waves were made in 2017 when LIGO–Virgo spotted a signal from the merger of two neutron stars.
A new cosmic messenger
This is not the first time that LIGO–Virgo has reported preliminary evidence of a black-hole–neutron-star merger. Earlier this year on 26 April the detectors recorded a possible signal from such an event, which may have occurred 1.2 billion light-years away. The signal from this object, however was too weak to confirm.
After upgrades that started in mid-2017, the LIGO and Virgo detectors were back in operation on 1 April 2019. By the end of July, they had spotted 18 binary black hole merger candidates and four binary neutron star merger candidates. However, unlike the neutron star merger of 2017 , no accompanying electromagnetic radiation from the four neutron-star mergers has been detected. Work is ongoing to confirm whether any of these candidates can be considered full-fledged discoveries.
