We Might Have Seen The First Gravitational Waves From A Neutron Star Colliding With A Black Hole

Artist's impression of a black hole colliding with a neutron star. Shutterstock/Kostafly

The third run of gravitational wave observations by the LIGO-Virgo network, which started on April 1, has already produced some stellar results. The observatories not only spotted a candidate neutron star collision on April 25, but they might also have observed a neutron star colliding with a black hole.

This could be the first discovery of gravitational waves from such an event. Along with these two candidate detections the collaboration has detected three more black hole mergers. This brings the total of confirmed and potential detections of gravitational waves to 16 since the first detection in 2015.

"The universe is keeping us on our toes," Patrick Brady, spokesperson for the LIGO Scientific Collaboration and a professor of physics at the University of Wisconsin-Milwaukee, said in a statement. "We're especially curious about the April 26 candidate. Unfortunately, the signal is rather weak. It's like listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this candidate."

The April 25 event (S190425z) was only seen by two detectors, which makes it difficult to pinpoint in the sky. This makes follow-up observations with conventional telescopes very unlikely. The April 26 possible merger, referred to as S190426c, was detected by all three instruments and researchers were able to narrow it down to a location within 3 percent of the area of the sky. Still sizable but possibly enough to discover a light-emitting counterpart.

“The first month of LIGO-Virgo’s third observing run has been truly amazing, potentially bringing us the discovery of the so far missing third, hybrid class of mergers – a neutron star merging with a black hole," said Professor Vicky Kalogera, Director of the Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University and member of the LIGO Scientific Collaboration, in an emailed statement. "Our multi-messenger team at Northwestern has been kept on its feet, and it is a treasure to be in this intellectual environment that combines both sides of the data analyses and the computer simulations of potential sources to understand what these signals are telling us.” 

The collaboration has issued public alerts regarding S190425z and S190426c, which have been picked up by both electromagnetic and astro-particle observatories on the ground and in space. No light-emitting counterpart has been identified at this stage but the search continues.

“It has been a truly spectacular start of the ‘hunting season’ for light from gravitational-wave sources. Our team is ready to repoint the Keck telescopes, among the largest on the globe," Assistant Professor Raffaella Margutti, also at Northwestern, explained. "We are still searching for any photon produced by the two mergers detected last month by LIGO-Virgo that might be able to reach our detectors on Earth and in space.” 

LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory, is made of two L-shaped 4-kilometer-long (2.5-mile) detectors, one in Louisiana and one in Washington State. Virgo is similar in structure but slightly smaller and is located in Italy. If you want to be among the first to know about possible gravitational wave detections, there is an app available for iPhone.

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