Last year researchers detected the first neutron star collision. It was an epochal discovery. The event was detected with both traditional observatories (from radio waves to gamma rays) and gravitational wave observatories, the first observation of its kind. Among the confirmation of many hypotheses, the merger also started a mystery. What kind of object was formed in this collision?

Based on its mass, it could be either a black hole or a neutron star. This summer, a team claimed that the merger most likely created a black hole. But a new analysis of the gravitational wave detection seems to suggest that the resulting object is a hypermassive neutron star. This is reported in the Monthly Notices of the Royal Astronomical Society.

Researchers Maurice van Putten of Sejong University in South Korea and Massimo Della Valle of Italy's Osservatorio Astronomico de Capodimonte looked at the data collected by the gravitational wave observatories and noticed that after the distinct “chirp” of the merger, there was a descending signal. This signal is consistent with a neutron star but not with a black hole.

“We’re still very much in the pioneering era of gravitational wave astronomy. So it pays to look at data in detail," van Putten said in a statement. "For us this really paid off, and we’ve been able to confirm that two neutron stars merged to form a larger one.” 

The neutron star formed in this collision is 2.7 times the mass of our Sun, close to the possible upper limit of how big neutron stars can get before they collapse into black holes. Another neutron star, known as PSR J1748-2021B, has a similar size and the two scientists think that it might have had the same origin.

Previous work has looked at the light emitted by the object, which was not as bright as it should be if the object was a neutron star. Considering the mass and light profile, a black hole seemed to be the best bet. This study turns this picture on its head but clearly leaves us with more questions.

Neutron stars and black holes are complex objects and it is likely that we are missing several important factors to do with how they behave and evolve. More observations are necessary to strengthen our understanding of neutron star collisions. The three gravitational wave observatories – the two LIGO facilities and the Virgo interferometer – will soon be back online after a technical shutdown and next year they will be accompanied by the Kamioka Gravitational Wave Detector (KAGRA) in Japan. This is only just the beginning of gravitational wave astronomy.