University of Glasgow: First Detection Of Gravitational Waves From Black Holes Swallowing Neutron Stars

For the first time, scientists have picked up the ripples in space-time caused by the death spiral of a neutron star and a black hole.

University of Glasgow researchers played a key role in the international collaboration that made the detection possible. They contributed to the design of the detectors – the most sensitive scientific instruments ever built – and the advanced data analysis needed to provide an astrophysical interpretation of the signals.

Gravitational waves are produced when celestial objects collide and the ensuing energy creates ripples in the fabric of space-time which carry all the way to the detectors we have here on Earth.

On 5 January 2020, the Advanced LIGO detector in Louisiana in the US and the Advanced Virgo detector in Italy, observed gravitational waves from this entirely new type of astronomical system.

LIGO and Virgo picked up the final throes of the death spiral between a neutron star and a black hole as they circled ever closer and merged together. They named the signal GW200105.

Remarkably, just days later, a second signal was picked up by Virgo and both ALIGO detectors – in Louisiana and Washington state – again coming from the final orbits and smashing together of another neutron star and black hole pair. This signal was named GW200115.An artists’ impression of the gravitational waves created from the death spiral of a neutron star and a black hole.

Scientists have long searched for neutron star–black hole binaries. Previous gravitational wave detections have revealed a population of colliding black holes, and neutron stars merging, but the mixing neutron star–black hole binaries have proved elusive. Past observations have provided enticing hints, but no conclusive observations.

The previously announced GW190814 might correspond to such a binary, but the unexpected mass of the lighter component is more likely a black hole than a neutron star. The candidate signal GW190426_152155 could be a neutron star–black hole, but is sufficiently quiet that it is not clearly distinguishable from noise. The new signals are more confident detections, and clearer observations of neutron star–black hole binaries.

Dr Christopher Berry, of the University’s School of Physics and Astronomy and the Center for Interdisciplinary Exploration and Research in Astrophysics, Northwestern University, said: “The detection of neutron star–black hole binaries represents the culmination of many decades of work developing gravitational-wave detectors. Thanks to their improvements in our instruments, we are able to detect signals from ever greater distances, boosting our detection rate.

“As we build a larger catalogue of gravitational-wave signals, we start to uncover a greater variety of sources. This diversity provides us with an unprecedented insight into the properties of black holes and neutron stars.

“Using measurements of the masses, spins and the rate of mergers, we can dig into the details of how massive stars evolve. Understanding how binary star systems work is one of the most important questions in astronomy. Fitting all the information from gravitational-wave observations will allow us to pin down the details of binary evolution. In the coming years, future observing runs will provide hundreds of detections, enabling precision measurements of how massive stars live their lives.”

Professor Sheila Rowan, director of the University of Glasgow’s Institute for Gravitational Research, said: “We’ve made a remarkable amount of progress since the historic first detection of gravitational waves in 2015, initiating the age of gravitational wave astronomy. This first detection of the collision of a neutron star and a black hole demonstrates that we have many new discoveries to look forward to, as well as new signals that will help deepen our understanding of the universe.

“Between now and summer next year, the LIGO detectors are set to be upgraded again, using what we’ve learned over previous observing runs to make them even more sensitive to the vibrations of spacetime. We’re looking forward to what we can learn from future detections.”

For decades, astronomers have predicted and theorised that this type of system – a black hole and neutron star merger – could exist, but without any convincing observational evidence.

Now gravitational wave scientists have finally witnessed the existence of this system they can start to shed more light on the birth, life and death of stars, as well how they form.

Dr Berry added: “While the masses of GW200105 and GW200115’s sources match our expectations for neutron star–black hole binaries, GW200115 has an interesting spin measurement. The spin of its black hole is most probably misaligned with the orbital angular moment. This type of misalignment is not typically expected, and could hint at something about how the system formed.

“Perhaps the binary formed dynamically in a dense environment like a globular cluster when a black hole and a neutron star strayed close together, rather than forming from two stars that lived their lives together? Or perhaps this is showing how supernova explosions can misalign black hole spins? Or perhaps this is a fluke measurement, and spins are typically aligned? Future observations of neutron star–black hole binaries will let us tease these possibilities apart.”

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