Black Hole and Neutron Star Collisions Detected Twice in Ten Days

Two instances of black hole-neutron star collision events have been detected using the Advanced LIGO and Virgo gravitational wave detectors, details of which have been published today in Astrophysical Journal Letters.

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.

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.

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.

During its third observing run, the LIGO–Virgo GW detector network observed GW200105 and GW200115, two GW events consistent with NSBH coalescences. Event GW200105 is effectively a single-detector event observed in LIGO Livingston with an S/N of 13.9. It clearly stands apart from all recorded noise transients, but its statistical confidence is difficult to establish.

Astrophysical Journal Letters – Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences

We report the observation of gravitational waves from two compact binary coalescences in LIGO’s and Virgo’s third observing run with properties consistent with neutron star–black hole (NSBH) binaries. The two events are named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and GW200115; the first was observed by LIGO Livingston and Virgo and the second by all three LIGO–Virgo detectors. The source of GW200105 has component masses $8.{9}_{-1.5}^{+1.2}$ and $1.{9}_{-0.2}^{+0.3}\,{M}_{\odot }$, whereas the source of GW200115 has component masses $5.{7}_{-2.1}^{+1.8}$ and $1.{5}_{-0.3}^{+0.7}\,{M}_{\odot }$ (all measurements quoted at the 90% credible level). The probability that the secondary’s mass is below the maximal mass of a neutron star is 89%–96% and 87%–98%, respectively, for GW200105 and GW200115, with the ranges arising from different astrophysical assumptions. The source luminosity distances are ${280}_{-110}^{+110}$ and ${300}_{-100}^{+150}\,\mathrm{Mpc}$, respectively. The magnitude of the primary spin of GW200105 is less than 0.23 at the 90% credible level, and its orientation is unconstrained. For GW200115, the primary spin has a negative spin projection onto the orbital angular momentum at 88% probability. We are unable to constrain the spin or tidal deformation of the secondary component for either event. We infer an NSBH merger rate density of ${45}_{-33}^{+75}\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$ when assuming that GW200105 and GW200115 are representative of the NSBH population or ${130}_{-69}^{+112}\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$ under the assumption of a broader distribution of component masses.

SOURCES- Cardiff, Astrophysical Journal Letters
Written By Brian Wang,

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