The merger of two neutron stars detected in 2017 probably generated a black hole

GW170817 seen by XMM-Newton (Image ESA/XMM-Newton; P. D'Avanzo (INAF–Osservatorio Astronomico di Brera))
GW170817 seen by XMM-Newton (Image ESA/XMM-Newton; P. D’Avanzo (INAF–Osservatorio Astronomico di Brera))

Two articles, one published in the journal “Astronomy and Astrophysics” and one in “The Astrophysical Journal Letter”, describe two researches on the consequences of the merger of two neutron stars detected last year at electromagnetic and gravitational waves. ESA’s XMM-Newton space telescope was used to monitor the evolution of its X-ray emissions. NASA’s Chandra X-ray observatory was similarly used and a team of researchers concluded that the merger generated a black hole.

The event indicated with as GW170817 caused an explosion called a kilonova, a concept that had been theoretical for a long time. There was still a lot to learn about the various phases of that event, with the merger of two neutron stars, the kilonova and the gamma-ray burst indicated with the acronym GRB 170817A but also on its consequences. The observations and the study of the data collected by very different instruments that allowed to cover the electromagnetic spectrum and gravitational waves keep on being analyzed in ongoing research.

A team of researchers led by Paolo D’Avanzo from INAF – Osservatorio Astronomico di Brera, Italy, author of the article published in “Astronomy and Astrophysics”, observed the decrease in the X-ray flow associated with the event GW170817. This is an important moment because the X-ray and radio-wave emissions continued for some months even growing in intensity, a surprising fact also because the optical and infrared frequency emissions decreased as expected.

On December 29, 2017, astronomers noticed the decrease in X-rays and radio waves thanks to the XMM-Newton space telescope, confirmed in the following weeks thanks to other independent observations in those bands of the electromagnetic spectrum. The monitoring continues to allow adding more data that are slowly forming a large cosmic portrait of the evolution of the event GW170817.

Regarding the consequences of that merger, David Pooley (Trinity University, San Antonio, Texas), Pawan Kumar (University of Texas at Austin), J. Craig Wheeler (University of Texas) and Bruce Grossan (University of California, Berkeley), authors of the article published in “The Astrophysical Journal Letter”, believe that the result is a black hole.

The researchers analyzed the data collected with the Chandra X-ray observatory after the event GW170817. The bottom image (NASA/CXC/Trinity University/D. Pooley et al. Illustration: NASA/CXC/M.Weiss) shows an artistic concept of the merger and observations of Chandra in August/September 2017 and December 2017.

If the merger of the two neutron stars had generated a more massive one, it would spin rapidly and generate a strong magnetic field. This, in turn, would create an expanding, highly charged particle bubble that would have generated strong X-ray emissions. Instead, Chandra detected much lower X-ray levels indicating that the result of the merger was a black hole.

The researchers also took into account observations made with the Very Large Array (VLA), which suggest that the X-ray emissions are due only to the shock wave caused by the merger of the two neutron stars. There are no traces of X-ray emissions due to a new neutron star and the radiation emissions following the merger were probably generated by a black hole.

It’s possible that other studies will provide other important conclusions about the GW170817 event and its consequences. Its detection was the first ever obtained in an extraordinary collaboration among space agencies and scientific entities all over the world. The observations continue and will help develop gravitational wave astronomy to discover new secrets of the cosmos.


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