A merger of two neutron stars observed at both electromagnetic and gravitational waves

The galaxy NGC 4993 seen from several different ESO telescopes (Image VLT/VIMOS. VLT/MUSE, MPG/ESO 2.2-metre telescope/GROND, VISTA/VIRCAM, VST/OmegaCAM)
The galaxy NGC 4993 seen from several different ESO telescopes (Image VLT/VIMOS. VLT/MUSE, MPG/ESO 2.2-metre telescope/GROND, VISTA/VIRCAM, VST/OmegaCAM)

Yesterday ESO and LIGO/VIRGO collaboration held a press conference to present the results of a complex research that led to the discovery of the merger of two neutron stars observed in the emission of both electromagnetic and gravitational waves. These findings were collected in a series of articles that were published or will be published in the magazines “Nature”, “Nature Astronomy”, “Astrophysical Journal Letters” and “Physical Review Letters”.

The event at the center of this research, called GW170817, was detected on August 17, 2017 in various ways by a variety of very different instruments. The two LIGO detectors had already been the focus of previous results in research on gravitational waves starting with the february 2016 annoucement. In August the VIRGO detector in Italy, near Pisa, was also activated with the result that all of them picked up the arrival of gravitational waves and working together they could pinpoint their origin.

The big difference from the previous gravitational wave detections is that ESA’s Integral and NASA’s Fermi space telescopes almost simultaneously detected a short-duration gamma-ray burst. As a result, ESO pointed various telescopes such as the Visible Survey Telescope for Astronomy (VISTA), the VLT Survey Telescope (VST), the Rapid Eye Mount (REM) and others towards the sky area from which the electromagnetic and gravitational signals came.

In the following days, ESO involved even more telescopes such as the Very Large Telescope (VLT), the New Technology Telescope (NTT), the VST and the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope. The MeerKAT radio telescope, a precursor of the next-generation SKA radio telescope, was also pointed to that sky area in the days following the detection.

Eventually, the origin of the event GW170817 was identified in the galaxy NGC 4993, about 130 million light years from Earth. The previous events detected at gravitational waves were most likely pairs of black holes that had merged but in this case the origin was quite different. Looking at all the data collected in various electromagnetic frequencies that were added to those that triggered the research, the scientists involved concluded that the source was a pair of neutron stars that merged in a kilonova.

Black hole mergers produced gravitational signals lasting just a few seconds, the event GW170817 generated a very different signal that lasted about one minute. There were some objections to the interpretations of the first gravitational signal detected by LIGO also because of its short duration, in this case it seems difficult to argue that a long signal such as the one from the event GW170817 might leave any doubts about its nature.

The kilonova was a theoretical concept of which this is the first observation, one of the reasons why it’s a great discovery. The consequences of that kind of event are the generation of heavy elements that are ejected into space. This means that this kind of research can really tell us where elements such as gold were created.

The kilonova also generated radioactive elements that left traces in the electromagnetic spectrum. They’re also among those observed by several telescopes during the observations that have been going on for many days to follow the consequences of that catastrophic event.

In cases of black hole mergers, gravitational detections were the only source of data. A new branch of astronomy was opened but seemed separate from the existing ones. In this case, there were observations in the electromagnetic spectrum as well in a research that integrated old and new astronomy.

The LIGO/VIRGO collaboration worked on this research together with other agencies and institutions around the world. This is another reason why it’s a truly extraordinary study that shows how an international collaboration is possible and can give wonderful results.

Artist's concept of a neutron star marger (Image University of Warwick/Mark Garlick)
Artist’s concept of a neutron star marger (Image University of Warwick/Mark Garlick)

This caltech video shows the various gravitational signals detected.

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