CITA Researchers Spearhead Astrophysical Interpretation of Record-Breaking Gravitational Wave Run
CITA’s GW Group, from left to right: Reed Essick (holding Milo), Maya Fishbach, Aryanna Schiebelbein-Zwack, Aditya Vijaykumar, Utkarsh Mali.
In August 2025, the global network of gravitational-wave observatories operated by the LIGO-Virgo-KAGRA (LVK) Collaboration released a huge new logbook of cosmic collisions – its fourth Gravitational-Wave Transient Catalog (GWTC-4.0) [1-6]. These gravitational waves, or ripples in spacetime, are created when the most extreme objects in the Universe, black holes and neutron stars, crash into each other. This latest update adds 128 new gravitational-wave candidates, more than doubling the count of logged events to date.
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.
Aryanna’s plot uses a computational tool that was developed by CITA faculty Prof. Reed Essick and is central to the analysis and interpretation of the data in GWTC-4.0. In addition to the masses of the bodies, the tool developed by Prof. Essick and his team accounts for additional stellar properties such as spins and binaries’ orbital inclinations.
To draw accurate conclusions, scientists must first understand the limitations of their observations. Think of the detectors as a fishing net. The net might be better at catching big fish than small ones. If you don’t account for that, you might wrongly conclude that no small fish exist. Essick and his team, including Mali, build the “instruction manual” [2] for this cosmic net, calculating which types of signals the detectors are most sensitive to.
“This data release, led by CITA faculty and students, provides the most robust estimates of the observable horizon to date,” Essick explains. “Thanks to the software we developed, we can directly measure the effects of stellar properties while making very few approximations. CITA’s work supports gravitational-wave astronomy not just in Canada, but across the entire world, inside and outside of the LIGO-Virgo-KAGRA collaborations.”
Together with the LVK, CITA’s faculty, postdocs, and students continue to provide world-leading computational tools and astrophysical insights into the lives and deaths of massive stars and their compact remnants, increasing our understanding of how they form and their connections to the broader universe.
[1] GW231123: a Binary Black Hole Merger with Total Mass 190-265 M⊙, https://arxiv.org/abs/2507.08219
[2] Compact Binary Coalescence Sensitivity Estimates with Injection Campaigns during the LIGO-Virgo-KAGRA Collaborations’ Fourth Observing Run, https://arxiv.org/abs/2508.10638
[3] GWTC-4.0: Population Properties of Merging Compact Binaries, https://arxiv.org/abs/2508.18083
[4] GWTC-4.0: Updating the Gravitational-Wave Transient Catalog with Observations from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run, https://arxiv.org/abs/2508.18082
[5] GWTC-4.0: Methods for Identifying and Characterizing Gravitational-wave Transients, https://arxiv.org/abs/2508.18081
[6] GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog, https://arxiv.org/abs/2508.18080
Contact:
Lyuba Encheva
Communications and Events Coordinator
Canadian Institute for Theoretical Astrophysics
Email: communication@cita.utoronto.ca