Black hole and neutron star merger detected for first time

Scientists have, for the first time, picked up the ripples in space-time caused by the collision of a neutron star and a black hole.

Two instances of this violent cosmic event have been detected using the Advanced LIGO and Virgo gravitational wave detectors, details of which have been published today in Astrophysical Journal Letters.

Though previous gravitational wave detections have spotted black holes colliding, and neutron stars merging, this is the first time that scientists have detected a collision from one of each.

Dr Vivien Raymond, from Cardiff University’s Gravity Exploration Institute, said: “After the detections of black holes merging together, and neutron stars merging together, we finally have the final piece of the puzzle: black holes swallowing neutron stars whole. This observation really completes our picture of the densest objects in the universe and their diet.”

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

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

The detectors 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.

Remarkably, on 15 January, 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.

Researchers from Cardiff University, who form part of the LIGO Scientific Collaboration, played a crucial role in the data analysis of both events, unpicking the gravitational wave signals and painting a picture of how the extreme collisions played out.

This involved generating millions of possible gravitational waves and matching them to the observed data to determine the properties of the objects that produced the signals in the first place, such as their masses and their location in the sky.

From the data they were able to infer that the first signal, dubbed GW200105, was caused by a 9-solar mass black hole colliding with a 1.9-solar mass neutron star.

Dr Charlie Hoy, a postdoctoral researcher in Gravity Exploration Institute, who also contributed to the analysis, said: “Although GW200105 was found in only one detector, we are confident that it is a real event and not just random noise from our detectors. It passed all of our quality checks and its shape is unlike random noise artefacts that we have seen before.”

Analysis of the second event, GW200115, which was detected just 10 days later, showed that it came from the merger of a 6-solar mass black hole with a 1.5-solar mass neutron star, and that it took place at a slightly larger distance of around 1 billion light-years from Earth.

“Virginia D’Emilio, one of my PhD students in the Gravity Exploration Institute, was a leading contributor to the inference of the sources’ properties, thus confirming their neutron-star and black-hole nature, and a member of the team leading the publication. This was a challenging and critical investigation with far reaching impact in the field," continued Dr Raymond.

For several years, since the first ever direct detection of gravitational waves in 2015, astronomers have predicted that this type of system – a black hole and neutron star merger – could exist, but without any compelling observational evidence.

Now that gravitational wave scientists have finally witnessed the existence of this new type of system, their detection will bring important new clues about how black holes and neutron stars form.

This will be helped by a new £9.4m grant for gravitational wave research awarded to UK universities and institutes by the STFC, £3m of which will go to Cardiff University over the next three years.

Professor Stephen Fairhurst, Director of the Gravity Exploration Institute at Cardiff University said: “The LIGO and Virgo detectors will begin a fourth data-taking run in the summer of 2022, with an increased sensitivity. During the run, we are very likely to observe additional mergers of Neutron Stars and Black Holes and gain a better understanding of how these systems form.

“Cardiff University has recently been awarded £3m of funding which will enable us to continue to play a leading role in identifying and understanding gravitational wave signals in the coming observing runs.”