Scientists present largest number of gravitational wave detections to date from black holes and neutron stars

Graphic depicting the gravitational wave mergers detected since the historic first discovery in 2015. Credit: Carl Knox (OzGrav, Swinburne University of Technology).

An international team of scientists, including Australian researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), have collaborated on a study released today, presenting the largest number of gravitational wave detections to date - 90 detections!

Gravitational waves are cosmic ripples in space and time that are caused by some of the most violent and energetic processes in the Universe, like supernovas, merging black holes and colliding neutron stars--city-size stellar objects with a mass about 1.4 times that of the Sun.

The newest gravitational wave detections come from the second part of the third observing run which lasted from November 2019 to March 2020. There were 35 new gravitational wave detections in this period: 32 detections were from pairs of merging black holes; 3 were likely to come from the collision of a neutron star and a black hole.

The gravitational-wave Universe is teeming with signals produced by merging black holes and neutron stars. In a new paper released today, an international team of scientists, including Australian OzGrav researchers, present 35 new gravitational wave observations, bringing the total number of detections to 90!

All of these new observations come from the second part of observing run three, called “O3b”, which was an observing period that lasted from November 2019 to March 2020. There were 35 new gravitational wave detections in this period. Of these, 32 are most likely to come from pairs of merging black holes, 2 are likely to come from a neutron star merging with a black hole, and the final event could be either a pair of merging black holes or a neutron star and a black hole. The mass of the lighter object in this final event crosses the divide between the expected masses of black holes and neutron stars and remains a mystery.

“It’s fascinating that there is such a wide range of properties within this growing collection of black hole and neutron star pairs”, says study co-author and OzGrav PhD student Isobel Romero-Shaw (Monash University). “Properties like the masses and spins of these pairs can tell us how they’re forming, so seeing such a diverse mix raises interesting questions about where they came from.”

Not only can scientists look at individual properties of these binary pairs, they can also study these cosmic events as a large collection - or population. “By studying these populations of black holes and neutron stars we can start to understand the overall trends and properties of these extreme objects and uncover how these pairs came to be” says OzGrav PhD student Shanika Galaudage (Monash University) who was a co-author on a companion publication released today: ‘The population of merging compact binaries inferred using gravitational waves through GWTC-3 P2100239’. In this work, scientists analysed the distributions of mass and spin and looked for features which relate to how and where these extreme object pairs form. Shanika adds, “There are features we are seeing in these distributions which we cannot explain yet, opening up exciting research questions to be explored in the future”.

Highlights

Of these 35 new events, here are some notable discoveries (the numbers in the names are the date and time of the observation):

  • Two mergers between possible neutron star - black hole pairs. These are called GW191219_163120 and GW200115_042309, the latter of which was previously reported in its own publication. The neutron star in GW191219_163120 is one of the least massive ever observed.
  • A merger between a black hole and an object which could either be a light black hole or a heavy neutron star called GW200210_092254
  • A massive pair of black holes orbiting each other, with a combined mass 145 times heavier than the Sun (called GW200220_061928)
  • A pair of black holes orbiting each other, in which at least one of the pair is spinning upright (called GW191204_171526)
  • A pair of black holes orbiting each other which have a combined mass 112 times heavier than the Sun, which seems to be spinning upside-down (called GW191109_010717)
  • A ‘light’ pair of black holes that together weigh only 18 times the mass of the Sun (called GW191129_134029)

The different properties of the detected black holes and neutron stars are important clues as to how massive stars live and then die in supernova explosions.

Detecting and analysing gravitational-wave signals is a complicated task requiring global efforts. Initial public alerts for possible detections are typically released within a few minutes of the observation. Rapid public alerts are an important way of sharing information with the wider astronomy community, so that telescopes and electromagnetic observatories can be used to search for light from merging events - for example, merging neutron stars can produce detectable light.

All of these detections were made possible by the global coordinated efforts from the LIGO (USA), Virgo (Italy) and KAGRA (Japan) gravitational-wave observatories.

The LIGO and Virgo observatories are currently offline for improvements before the upcoming fourth observing run (O4), due to begin in August 2022 or later. The KAGRA observatory will also join O4 for the full run. More detectors in the network help scientists to better localise the origin or potential sources of the gravitational waves.

CONTACTS FOR INTERVIEW

Isobel Romero-Shaw

OzGrav PhD student, Monash University

isobel.romero-shaw@monash.edu

Phone number: +61 487 033 130

Shanika Galaudage

OzGrav PhD student, Monash University

shanika.galaudage@monash.edu

Phone number +61 408 093 669

Additional information about the gravitational-wave observatories:

The ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) is funded by the Australian Government through theAustralian Research Council Centres of Excellence funding scheme. OzGrav is a partnership betweenSwinburne University of Technology (host of OzGrav headquarters), theAustralian National University,Monash University,University of Adelaide,University of Melbourne, andUniversity of Western Australia, along with other collaborating organisations in Australia and overseas.

This material is based upon work supported by NSF’s LIGO Laboratory which is a major facility fully funded by the National Science Foundation. NSF’s LIGO Laboratory is a major facility fully funded by the National Science Foundation and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. 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. Nearly 1300 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 athttps://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 700 members from 125 institutions in 15 different countries. 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 members can be found athttp://public.virgo-gw.eu/the-virgo-collaboration/.


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