The large number of events allows scientists to move beyond studying individual collisions and begin to chart a bigger picture. By creating a “statistical census” of colliding black holes and neutron stars (measuring their sizes, spins, and locations), the researchers can uncover their origin stories and figure out how the universe builds these extreme pairs.

At the heart of the latest LVK’s data release is a team of CITA researchers, some of whom have been involved with gravitational-wave astrophysics since the very first detection of a binary black hole in 2015. CITA faculty Prof. Maya Fishbach leads the Rates and Populations Working Group within the LVK, which is responsible for providing astrophysical interpretations of the Gravitational-wave data collected by the 4 detectors (LIGO Hanford and Livingston, Virgo, and KAGRA). She reports, “we have observed 128 new gravitational-wave candidates, more than doubling the size of the previous catalog. Most of the events are from merging binary black holes and include the most massive gravitational-wave merger observed to date and the loudest gravitational-wave signal detected.” [1, 4]

Dr. Aditya Vijaykumar, a CITA Postdoctoral Fellow, led a major paper [3] that summarizes what this new flood of data tells us about the “family” of black holes and neutron stars in our universe.

“We can now measure the universe’s population of black holes and neutron stars in better detail than ever before,” he says. “We can see how often they merge, the full range of their sizes, and how they spin.”

These details are like fingerprints, revealing the “life stories” of these extreme objects. “Features in the data—like gaps or pile-ups in the range of black hole sizes, the direction they spin, and hints of different sub-groups—contain clues about how they were born and evolved over cosmic history,” Vijaykumar explains.

CITA graduate student Utkarsh Mali, together with incoming CITA Postdoctoral Fellow Dr. Amanda Farah, analyzed the entire mass, spin, and redshift distribution of neutron stars and black holes simultaneously as part of the paper [3] led by Dr. Vijaykumar. Their analysis improves previous measurements of the rates of the three types of gravitational-wave mergers: binary neutron stars, binary black holes, and mixed neutron star–black hole mergers. Former University of Toronto undergraduate student Tom Wu, now a graduate student at the University of North Carolina, also provided an updated calculation of the merger rate of binary neutron stars, which has dropped compared to previous catalogs. Understanding the merger rates of neutron stars and black holes is a critical component to unveiling their mysterious origins.

Graduate student Aryanna Schiebelbein-Zwack contributed to the main catalog paper [4] by computing the cosmic reach of the data set. The plot she created shows how the observable horizon depends on the masses of the bodies in the binary collisions.

Hearing the forest for the trees

Ben Farr (University of Oregon) — December 9, 2024

Abstract

Before the start of their current and fourth observing run (O4), the LIGO and Virgo detectors had already collected an impressive census of compact binary mergers in the local universe. By the end of O2 in August 2017 the LIGO Scientific Collaboration and Virgo Collaboration claimed a total of 10 binary black hole mergers and one binary neutron star merger. O3 spanned April 2019 through March 2020, included many significant discoveries (e.g., neutron star-black hole binaries, surprisingly massive black holes), and culminated in the third catalog of compact binary mergers, GWTC-3, raising the total catalog of confidently detected binary mergers to 90. I will present some of what ground-based gravitational wave astronomy has taught us about compact binaries over the last nine years, and what may lie ahead.

The LIGO-Virgo-KAGRA (LVK) Collaboration released information on one of the events detected during its current fourth observing run – a gravitational-wave signal from the collision of what is most likely a neutron star with a compact object that is 2.5 to 4.5 times the mass of our Sun.

What makes this signal, called GW230529, intriguing is the mass of the heavier object. It falls within a possible mass-gap between the heaviest known neutron stars and the lightest black holes. The gravitational-wave signal alone cannot reveal the nature of this object. Future detections of similar events, especially those accompanied by bursts of electromagnetic radiation, could hold the key to solving this cosmic mystery.

“GW230529 straddles a particularly interesting range of masses where we expect there could be both neutron stars and black holes. This signal’s astrophysical interpretation therefore requires knowledge of nuclear physics and the population of compact objects. CITA has expertise in both these areas, and our analyses guide the interpretation of this signal,” says CITA’s , who led the analysis to determine how likely it is that GW230529’s source contained a black hole.

CITA faculty and Reed Essick,��CITA Fellows and , and University of Toronto graduate students Utkarsh Mali and Aryanna Schiebelbein-Zwack have a longstanding involvement with the LVK, with Essick and Fishbach being part of the international collaboration since the detection of its very first significant events. To this day, gravitational-wave observations have observed around 100 mergers of compact objects. Out of these, only one other merger involved a mass-gap compact object similar to the one in event GW230529.

Analysis of signal GW230529 shows that it came from the merger of two compact objects, one with a mass between 1.2 to 2.0 times that of our Sun and the other up to twice as massive. While the gravitational-wave signal does not provide enough information to determine with certainty whether these compact objects are neutron stars or black holes, it seems likely that the lighter object is a neutron star and the heavier object a black hole.

Interpreting these signals relies on models of the astrophysical population of merging compact objects and the behaviour of extremely dense matter like that found in the cores of neutron stars. As such, it brings together expertise from the LVK’s Extreme Matter and Rates & Populations working groups, which are co-chaired by Dr. Landry and Prof. Fishbach, respectively.

Nuclear physics and general relativity limit the mass and spin that neutron stars can achieve. Based on previous work with Dr. Landry () and Prof. Fishbach (), Prof. Essick combined our current understanding of high-density nuclear physics and the observed population of merging binaries to compute the probability that GW230529 is consistent with these bounds, finding that there is at least a 70% chance that GW230529’s source contained a black hole.

“Gravitational waves continue to surprise us with discoveries of black holes and neutron stars with unexpected masses, unlike any of the compact objects that we see in our galaxy,” says Prof. Fishbach. “Understanding why the population of gravitational-wave sources differs from the Galactic population of compact objects can help us learn about the diverse formation histories of these systems.”

According to Dr. Landry, “previous gravitational wave and electromagnetic observations have hinted at the existence of compact objects in the 2 to 5 solar mass range, but this discovery is exciting because it is the first time we see a compact object from the purported mass gap paired with a neutron star. Gravitational waves are telling us that the gap is not empty.”

The fourth observing run is planned to last for 20 months including a couple of months break to carry out maintenance of the detectors and make a number of necessary improvements. By January 16, 2024, when the commissioning break started, a total of 81 significant signal candidates had been identified. GW230529 is the first of these to be published after detailed investigation. While the observing run continues, LIGO-Virgo-KAGRA researchers are analyzing the data from the first half of the run and checking the remaining 80 significant signal candidates that have already been identified. By the end of the fourth observing run in February 2025, the total number of observed gravitational-wave signals should exceed 200.

Read the full article on the .

Gravitational-wave observatories:

LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,600 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are listed at .

The Virgo Collaboration is currently composed of approximately 880 members from 152 institutions in 17 different (mainly European) 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 the National Institute for Subatomic Physics (Nikhef) in the Netherlands. A list of the Virgo Collaboration groups can be found at: . More information is available on the Virgo website at .
KAGRA is the laser interferometer with 3 km arm-length in Kamioka, Gifu, Japan. The host institute is Institute for Cosmic Ray Research (ICRR), the University of Tokyo, and the project is co-hosted by National Astronomical Observatory of Japan (NAOJ) and High Energy Accelerator Research Organization (KEK). KAGRA collaboration is composed of over 400 members from 128 institutes in 17 countries/regions. KAGRA’s information for general audiences is at the website https://gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are accessible from .