Laboratoire d'Annecy de Physique des Particules
Science

GWTC-3 : a new batch of gravitational waves detections

From 1 April 2019 to 27 March 2020, the LIGO and Virgo gravitational wave detectors conducted a sky observation called O3. The last 6 months of this observation have detected 35 astrophysical events: 32 mergers of black hole pairs and 3 mergers of a black hole with a neutron star. These 35 new detections are described in the GTWC-3 catalog published on 8 November 2021.

From 1 April 2019 to 27 March 2020, the LIGO and Virgo gravitational wave detectors carried out a sky observation (called O3). The first 6 months of this observation period (O3a) provided 44 detected events that were included in the GWTC-2.1 catalogue published in July 2021 (https://arxiv.org/abs/2108.01045). The following 6 months (O3b) saw the accumulation of 35 additional events: 32 mergers of black hole pairs and 3 mergers of a black hole with a neutron star. These 35 new detections are described in the GTWC-3 catalogue published on 8 November 2021 (https://arxiv.org/abs/2111.03606).

Since 14 September 2015, when a gravitational wave was first detected, 90 events have been observed by the LIGO and Virgo interferometric detector network. All of them are the result of the merger of two black holes, two neutron stars or a black hole and a neutron star. [1]

The 3 neutron star-black hole mergers in the GWTC-3 catalogue have the following characteristics:

  • GW191219_163120 [2] is a highly asymmetric pairing of a 31.6 solar mass (31 Mo) black hole and a 1.17 solar mass (1.17 Mo) neutron star. The latter is the least massive neutron star observed to date.
  • GW200115_042309 is a neutron star (1.44 Mo) – black hole (5.9 Mo) pair already widely described in a scientific paper specifically dedicated to this detection published in June 2021 (https://iopscience.iop.org/article/…)
  • GW200210_092254 is a neutron star (2.79 Mo) – black hole (24.6 Mo) pair similar to the GW190814 pair detected in O3a for which it is not known if the compact object of 2.79 Mo is a very massive neutron star or a very light black hole

Within this GWTC-3 catalogue, some black hole pairs are also worth highlighting:

  • GW200220_061928 is probably the most massive pair of black holes observed during O3b: 86 Mo for one and 59 Mo for the other. Their merger formed a 141-Mb black hole, which belongs to the intermediate-mass black holes, i.e. between the black holes resulting from the death of a massive star (less than 100 Mo) and the super-massive black holes found at the centre of most galaxies (more than 1000000 Mo).
  • GW191129_134029 is the least massive pair of black holes observed in O3b: 10.7 Mo and 6.8 Mo
  • GW191204_171526 is a pair of black holes (11.9 Mo and 8.2 Mo) for which it could be estimated with quasi-certainty that at least one of them was rotating on itself, with an axis of rotation in the same direction as the orbital rotation axis of the two black holes. The first observation of such a system was GW151226, described in the GTWC-1 catalogue published in September 2019 (https://journals.aps.org/prx/abstra…).
  • GW191109_010717 is a similar system but more massive (65 Mo and 47 Mo) and with a rotation axis in the opposite direction to the orbital axis. It is quite rare to observe such astrophysical objects spinning in the opposite direction of their orbital rotation.

Of the 35 detections of O3b, 18 were detections made by the online analyses and were therefore the subject of an alert to the telescopes and satellites for a possible detection of an electromagnetic counterpart. None of these alerts were followed by such a detection. This highlights the exceptional luck that accompanied the detection of the merger of two neutron stars on 17 August 2017, the only one to date that was also seen by satellites and telescopes.

In addition to the 35 events that have been estimated to be statistically significant, there are 1048 candidates within GWTC-3 for which the probability that they are of astrophysical origin is lower. This additional list will allow future analyses to provide additional events or allow astronomers to search for optical counterparts a posteriori.

The GTWC-3 catalogue is accompanied by two other publications dealing with the cosmological and astrophysical consequences of these detections and a publication reviewing the coincidences between these gravitational wave events and the Gamma Ray Burst events observed during O3b by satellites. Finally, GWTC-3 is also the occasion to make public all the calibrated data recorded by LIGO and Virgo over the O3b period, between November 2019 and March 2020 (https://www.ligo.org/science/Public…)

Like the GWTC-2.1 catalogue, the GTWC-3 catalogue is the result of a huge amount of construction, detector tuning and data analysis work to which the Virgo team at LAPP has made a major contribution.

All data are available at https://www.gw-openscience.org and the GTWC-3 catalogue is published here: https://arxiv.org/abs/2111.03606

This image shows the 90 mergers of binary systems of compact stars that have been observed to date by the global array of Advanced LIGO and Advanced Virgo detectors. Each box represents one of these events, whose name is indicated at the bottom. The masses of the merging stars (black holes or neutron stars) and of the stars formed during these mergers are given in solar masses. The colour of each box indicates during which of the three data sets the signal was detected: O1 in 2015-2016, O2 in 2016-2017 and O3 in 2019-2020. The increase in the event rate during O3 is a consequence of the improved performance of the three detectors in the network as well as the data analysis methods. Note that the table actually contains 91 cells because it includes the GW200105 event, although the latter signal is considered marginal (i.e. its astrophysical origin is uncertain).

[1] Neutron star and black hole are two extreme results of the end of a star’s life.

[2] Each detected event is named according to its date year_month_day (yymmdd) and time hour_minute_second (hhmmss) of detection: GWyymmdd_hhmmss