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A possible fusion between a neutron star and a black hole observed for the first time

On April 26, 2019, a gravitational wave characteristic of the fusion of a black hole with a neutron star passed through the LIGO and Virgo detectors. This meeting, which is totally new, is not the only one that has been observed. While LIGO and Virgo have only been in working order for a month, after a period of work to make them more efficient, three black hole fusions and one neutron star fusion have also been detected.

In addition, for the first time, LIGO-Virgo alerts are public. They are sent quickly after real-time detection of possible gravitational wave signals. This strategy is intended to facilitate the follow-up of these observations by other observatories and further enhances the already promising prospects of multi-messenger astronomy, based on the simultaneous recording and interpretation of various signals from space.

"Dealing with three heterogeneous detectors is challenging work", says Florian Aubin, PhD student at the University Savoie Mont Blanc, France. "But it is also a great opportunity to identify the sky position of the source and search for electromagnetic / neutrino counterparts, that is to say, light or particle emissions. I am very excited for the coming run. These two NS merger candidates and the other three BBH merger candidates, in less than a month, promise a full year of interesting discoveries. It is really rewarding for me to be here, after two years of tough work."

The very high operating rate of the Advanced Virgo detector, combined with its geographical distance from the LIGO detectors and its different orientation, improves the ability of the global network to locate gravitational wave sources in the sky and to fully decrypt these signals. Indeed, a good determination of the position of the source in the sky is a prerequisite for a successful observation campaign of electromagnetic counterparts, and for the rich harvest of scientific results that follows.

Many detections are expected as the Advanced LIGO and Advanced Virgo instruments continue their new year-long observation campaign : new fusions of black holes and systems containing neutron stars of course, but also possibly new gravitational wave signals from other types of astrophysical sources. Both real-time detection algorithms, which analyze data once it is produced, and subsequent analyses, which process large amounts of data using all available information, will continue to search for gravitational waves, from known or unknown sources.

Illustration : Artist’s view of a fusion of neutron stars, in several stages. The result of the fusion is likely to become a black hole, no longer able to withstand its own gravity. Image Ciolfi, Giacomazzo (Virgo Collaboration), Kastaun (Ligo Scientific Collaboration)