Dozens of new gravitational-wave events

New LIGO/Virgo catalog contains 50 signals, several from unusual sources

The LIGO Scientific Collaboration and the Virgo Collaboration have published updates to their catalog of gravitational-wave signals, their astrophysical implications and more stringent tests of general relativity. The Gravitational-Wave Transient Catalog 2 (GWTC-2) now contains 50 signals compared to 11 signals in the previous version. The 39 new discoveries were found in O3a, the first six months of the third joint observing run “O3”, which began on 1 April 2019. The new signals come from different astrophysical systems of merging black holes and neutron stars in all possible combinations. Some exceptional events have already been published in the previous months. While the new catalog contains many binary black hole mergers found routinely, it also features some more surprises, such as the most light-weight merger of two black holes or a possible merger of a black hole and a neutron star. Results from the second half of O3 will be published later and should contain more surprises from the Dark Universe.

The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2 and O3a). The diagram shows black holes (blue), neutron stars (orange) and compact objects of unknown nature (grey), which were detected by their gravitational-wave emission. Each merger of a binary system corresponds to three compact objects shown: the two merging objects and the result of the merger. A selection of black holes (violet) and neutron stars (yellow) discovered by electromagnetic observations is shown for comparison.

Apart from the previously published exceptional events GW190412, GW190425, GW190521, and GW190814, the new catalog GWTC-2 includes two more especially noteworthy events, called GW190426_152155 and GW190924_021846 (in a new naming convention the UTC time of the detection is appended to the name).

“One of our new discoveries, GW190426_152155, could be a merger of a black hole of around six solar masses with a neutron star. Unfortunately the signal is rather faint, so we cannot be entirely sure,” explains Serguei Ossokine, a senior scientist at AEI Potsdam. “GW190924_021846 certainly is from the merger of the two lightest black holes we’ve seen so far. One had the mass of 6 Suns, the other that of 9 Suns. There are signals from mergers with less massive objects like GW190814 but we don’t know for sure whether these are black holes.”

Together with their Virgo colleagues, the AEI researchers Harald Lück, Moritz Mehmet, and Henning Vahlbruch install the squeezed-light source, which was developed in Hannover, in a cleanroom at the Virgo gravitational-wave detector.

“One key to finding a new gravitational-wave signal about once every five days over six months were the upgrades and improvements of the two LIGO detectors and the Virgo detector,” says Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) and director of the Institute for Gravitational Physics at Leibniz University Hannover. “Important roles played, for example, the high-power lasers developed at AEI Hannover, new mirrors, and the reduction of background noise sources. This increased the volume in which our detectors could pick up the signal from, say, merging neutron stars by a factor of four!”

How the raw data from the detectors are processed, how short disturbances (glitches) are dealt with, and how data are calibrated was also improved, allowing the LIGO/Virgo researchers to listen deeper into the cosmos than ever before.

“Our improvements of the waveform models we use to search for the signals in the detector data were crucial for the detections,” says Alessandra Buonanno, director at AEI Potsdam and professor at the University of Maryland. “We need accurate waveform models with full physical effects also to identify the different astrophysical sources, to understand the population of these sources in the Universe, and to pursue tests of gravity in the strong and dynamical regime.” She adds, “For the first time we analyzed in-depth the properties of the remnant object while it rings spacetime, and we studied the implications of higher harmonics in the signal on tests of general relativity.”

In O3a, the first six months of the third joint LIGO/Virgo observing run O3, the researchers found four times as many signals as in the two previous observing runs (11 months total observing time) combined. Their analysis has revealed 39 new events, increasing the total to 50. Preliminary information was reported on 26 of these signals as open public alerts in near real-time, while the remaining 13 were found by more sensitive and more time consuming follow-up searches. On closer inspection, 7 of the 33 open public alerts issued in O3a could not be identified as gravitational-wave signals.

“When you look at the catalog, there’s one thing all events have in common: They come from mergers of compact objects such as black holes or neutron stars. But if you look more closely, they all are quite different,” says Frank Ohme, leader of an Independent Max Planck Research Group at AEI Hannover. “We’re getting a richer picture of the population of gravitational-wave sources. The masses of these objects span a very wide mass range from about that of our Sun to more than 90 times that, some of them are closer to Earth, some of them are very far away.”

Visualization of the coalescence of two black holes that inspiral and merge, emitting gravitational waves. One black hole is 9.2 times more massive than the other and both objects are non-spinning.

While most of the new discoveries come from mergers of “ordinary” stellar-mass binary black holes, a few stand out as exceptional. Some of them have been published previously:

  • GW190412: the first observation of a binary black hole merger where the two black holes have distinctly different masses,
  • GW190425: most likely the second observation of a binary neutron star merger,
  • GW190521: the most massive binary black hole merger with a total mass of 150 Suns and the first observation of the birth of an intermediate-mass black hole
  • GW190814: the merger of a 23-solar mass black hole with a mysterious object 9 times lighter

The LIGO/Virgo researchers have published three papers accompanying their new catalog on the arXiv preprint server today. One tests how well the gravitational-wave events in the catalog agree with general relativity and finds no evidence for new physics beyond this theory. Another publication studies what the detected events can tell us about the Universe’s population of their sources. The third paper describes a search for gravitational waves that might be detectable in coincidence with gamma-ray bursts in the O3a period. No such signals were found.

AEI researchers have significantly contributed to analyses presented in the four papers. They have provided accurate models of the gravitational waves from coalescing black holes that included, for the first time, the precession of the black-holes’ spins, multipole moments beyond the dominant quadrupole, as well as tidal effects introduced by the potential neutron-star companion. Those features imprinted in the waveform are crucial to extract unique information about the source’s properties and carry out tests of general relativity. The high-performance computer clusters “Minerva” and “Hypatia” at AEI Potsdam and “Holodeck” at the AEI Hannover were employed in the development of the waveform models and their use in the events’ analyses.

The LIGO and Virgo researchers have issued open public alerts for another 23 possible gravitational-wave events (candidates) in O3b, the second half of O3, which lasted from 1 November 2019 to 27 March 2020. None of the candidates have been published as gravitational events. LIGO and Virgo scientists are examining all remaining candidates and will publish all those for which detailed follow-up analyses confirm their astrophysical origin.

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LIGO is funded by the National Science Foundation (NSF) and operated by Caltech and MIT, which conceived of LIGO and lead the project. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 550 members from 106 institutes in 12 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups can be found at http://public.virgo- gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at http://www.virgo-gw.eu.

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