In late April of 2019, two sets of gravitational waves observations were detected by a pair of American observatories as well as by one in Italy. The signals come from a neutron star merger, but the second set of signals suggests evidence of a black hole-neutron star collision, which scientists did not imagine was possible. 


Thanks to upgrades to the observatories, the improved sensitivity of the detectors allowed for a more accurate picture of the gravitational wave activity. On April 25, two neutron stars were detected smashing into each other about 500 million light-years away from Earth. Unfortunately, only two of the three observatories were able to detect this signal, and due to this lack of complete detection, it is unclear where the event originated.


The Laser Interferometer Gravitational-Wave Observatory (LIGO) based in Livingston, Louisiana, and Hanford, Washington, as well as the Virgo interferometer in Italy were able to detect activity on April 26 of a neutron star-black hole crash. Because all three observatories received the (albeit weak) signal, scientists were able to narrow down its location to a 3 percent area of the sky. 


Mass is the key distinction that helps us to distinguish whether the collision occurred between a neutron star and a black hole, two neutron stars, or two black holes. Neutron stars usually have a lower mass than black holes, and estimates from the gravitational wave signals that were measured fall into a range that is neither too light nor too heavy. There is still much to learn from the April 26th collision, such as what those two objects looked like before collision, whether they were actually a neutron star and a black hole, and what the result of the collision looks like now. 


At the moment, there are several theories as to how the event occurred. According to Shaon Ghosh, a member of the LIGO team and a postdoctoral research associate at the University of Wisconsin Milwaukee, one theory is of a system where two massive stars spend their lifetime together, evolve, and then form a neutron star and black hole. “These two compact objects emit gravitational waves, losing energy and angular momentum and shrinking the separation between them and eventually coalescing,” says Ghosh. Another theory involves an unrelated neutron star and a black hole accidentally getting too close to each other, interacting gravitationally until they eventually merge.