New Delhi: For a long time astronomers found that black holes merge with black holes and neutron stars merge with other neutron stars. But on January 5, 2020, astrophysicists heard a chirp from a distant part of the galaxy, approximately 900 million light-years away.
The transient sound was unlike any they had heard before and was caused by a great ripple in space-time -- a gravitational wave -- that spread out across the universe from over 900 million light-years away, washing over the Earth and pinging detectors.
Ten days later, astronomers heard a similar sound again. Gravitational waves had once again pinged Earth's detectors. After precise analysis, the two signals have been identified as the collision between a black hole and a neutron star which was something never happened before.
A new detailed study of the collision or merger was published in the Astrophysical Journal Letters on Tuesday (June 29), featuring over 1,000 scientists from the LIGO/Virgo and KAGRA collaborations, a multinational effort to hunt for gravitational waves. The two newly described events are named GW200105 and GW200115, for the dates they were discovered, and provide the first definitive evidence of an elusive merger.
"These are the first really confident detections of the merger of a neutron star with a black hole," adds Rory Smith, an astrophysicist at Monash University in Australia and a member of the LIGO collaboration.
Black holes and neutron stars are mysterious celestial objects which are formed from the death of a star. The size of a star affects how its life comes to a close. If it's a small star (small being "about 10 times more massive than our sun"), it collapses into an incredibly dense neutron star. If it's a big star, it collapses into a black hole. Both are well-known and well-studied objects, but they still contain many mysteries.
GW200105, the chirp detected on January 5, 2020, and GW200115, the chirp detected on January 15, 2020, are similar events but the objects that collided have slightly different properties.
The researchers say that GW200105 is the result of a black hole about nine times as massive as the sun colliding with a neutron star about 1.9 times as massive as the sun. GW200115 formed from a black hole about six times as massive as the sun merging with a neutron star about 1.5 times as massive as the sun. The mergers occurred almost a billion years ago far from the Earth, and the chirps only reached us recently.
There's also the possibility the black hole "shreds" the neutron star in a process known as tidal disruption. In this scenario, the black hole would rip material from the surface of the neutron star and steal it, creating a disk of debris around the event horizon. "That should produce an electromagnetic signal," Scott said.
A shredded neutron star is a goldmine for astrophysicists as one can't make the material present in a neutron star in a laboratory and study it, so these types of events may open a window to understanding what's happening inside them.
"By watching how a neutron star is pulled apart by a black hole, we are beginning to learn about how matter behaves in its most dense state," said Eric Thrane, an astrophysicist at Monash University and member of the LIGO collaboration. With enough gravitational wave detections, we may be able to decode their properties.
GW200105 and Carl GW200115 being the first of many black hole-neutron star mergers may help to explore the most extreme objects in our universe and may unveil new laws of nature one day.