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Artist visualization of a binary black hole merger with similar properties to the two new events.鈥� Artists Credit: Binary Black Hole Merger Carl Knox, OzGrav, Swinburne University of Technology

In a new published Oct. 28 in The Astrophysical Journal Letters, the international LIGO-Virgo-KAGRA (LVK) Collaboration reports on the detection of two gravitational wave events in October and November of last year with highly suggestive black hole spins and mass ratios.聽

Gravitational waves are 鈥渞ipples鈥� in space-time that result from cataclysmic events in deep space, with the strongest waves produced by the collision of black holes. The first of the two new mergers, GW241011 (Oct. 11, 2024), occurred roughly 700 million light years away and resulted from the collision of two black holes weighing in at around 20 and 6 times the mass of our sun. The larger object in GW241011 was measured to be one of the fastest rotating black holes observed to date.聽

Almost one month later, GW241110 (Nov. 10, 2024) was detected around 2.4 billion light years away and came from the merger of black holes roughly 17 and 8 times the mass of our sun. While most observed objects spin in the same direction as their orbit, the larger black hole of GW241110 was confidently found聽 to be spinning in a direction opposite its orbit 鈥� a first of its kind.聽

鈥淲e have had many 鈥榝irst detections鈥� of new types of systems over the past 10 years, and these new events just go to show that the Universe still has a lot to show us,鈥� says CITA鈥檚 Prof. Reed Essick. 鈥淟arger catalogs and improved detector sensitivities give us the best chance at understanding the full range of ways that black hole binaries are assembled, evolve, and merge.鈥�

Uncovering Hidden Properties of Black Hole Mergers

Gravitational waves were first predicted by Albert Einstein as part of his general theory of relativity in 1916, but their presence 鈥� though proven in the 1970s 鈥� wasn鈥檛 directly observed by scientists until just 10 years ago, when the LIGO observatory confirmed the detection of gravitational waves from the merger of two black holes.

Today, the LVK is a worldwide network of advanced gravitational-wave detectors and is in the midst of its fourth observing run, O4. The current run started in late May 2023 and is expected to continue through mid-November of this year. To date, approximately 300 black hole mergers have been observed through gravitational waves, most of which were identified during O4.

Together, the detection of GW241011 and GW241110 highlight the remarkable progress of gravitational-wave astronomy in uncovering the properties of merging black holes. Interestingly, both detected mergers point toward the possibility of 鈥渟econd-generation鈥� black holes. Such second-generation black holes have been predicted by the simulations of CITA Postdoctoral Fellows Claire Shi Ye, who simulates black holes in dense stellar clusters, and Vera Delfavero, who simulates black holes in the disks of active galactic nuclei.

“For the past decade, the holy grail of gravitational-wave astronomy has been to find out whether merging black holes lived their whole lives together in a stellar binary or whether they found each other later in life in a dense cluster of stars,鈥� CITA鈥檚 Prof. Maya Fishbach explains. 鈥淭hanks to these latest observations, we can finally say that some black holes indeed come from dense star clusters.”

The similarity of these latest observations to Dr. Ye鈥檚 is highlighted by certain clues, including the size differential between the black holes in each merger 鈥� the larger was nearly double the size of the smaller 鈥� and the spin orientations of the larger of the black holes in each event. A natural explanation for these peculiarities is that the black holes are the result of earlier coalescences. This process, called a hierarchical merger, suggests that these systems formed in dense environments, in regions like star clusters, where black holes are more likely to run into each other and merge again and again.

鈥淭hese detections are very exciting, as they offer valuable clues about the formation and evolution of some binary black hole mergers,鈥� said Dr. Claire Ye. 鈥淚t is elegant when observations and theoretical predictions, rooted in a deep understanding of dynamical evolution of dense environments and black hole formation, come together to reveal the origins of black holes.鈥�

鈥淚 was the expert on duty in-charge of estimating the properties of new signals when the November 10th event arrived. Its unusually high, well-measured spin鈥攐pposite to its orbital motion鈥攊mmediately caught my attention, especially given its similarity to the event just a month earlier,鈥� says CITA Postdoctoral Fellow Dr. Aditya Vijaykumar. 鈥淚 am very excited about the science these events will enable. Particularly, we know that some such mergers in dense crowds will have elliptical orbits instead of circular ones. Detecting these elliptical mergers in the future will open up an entirely new window into how these binaries formed and evolved.鈥�

CITA’s Contributions to LVK Data Releases (GWTC-4.0)

In the past few months, the global network of gravitational-wave observatories operated by the LVK Collaboration released a huge new logbook of cosmic collisions 鈥� its fourth Gravitational-Wave Transient Catalog (GWTC-4.0) [1-6]. This latest update adds 128 new gravitational-wave candidates from only the first third of O4, which nevertheless more than doubles 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 鈥渟tatistical census鈥� 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鈥檚 data releases 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).聽

A computational tool developed by CITA faculty Prof. Reed Essick was central to the analysis and interpretation of the data in GWTC-4.0. In addition to the objects鈥� masses it accounts for stellar properties such as spins and binaries鈥� orbital inclinations when predicting how likely they are to be detected. Prof. Essick also wrote the 鈥渋nstruction manual鈥� [2] for calculating which types of signals the detectors are most likely to recognise as significant events.

Dr. Aditya Vijaykumar, CITA Postdoctoral Fellow, is a key contributor to and co-author of a major paper [3] that summarizes what this new flood of data tells us about the 鈥渇amily鈥� of black holes and neutron stars in our universe. CITA graduate student Utkarsh Mali, together with CITA Postdoctoral Fellow Dr. Amanda Farah, analyzed the mass, spin, and redshift distribution of neutron stars and black holes simultaneously as part of the paper [3] led by Dr. Vijaykumar. CITA graduate student Aryanna Schiebelbein-Zwack also contributed by quantifying how much of the universe is surveyed by the current observations.

The LVK Group at CITA is continuously expanding, welcoming two new postdoctoral fellows, Dr. Vera Delfavero and Dr. Amanda Farah, this fall. We continue to look forward to many more breakthroughs from CITA and the LVK as the rest of the events from O4 are published in the coming years.

鈥淕W241011 and GW241110: Exploring Binary Formation and Fundamental Physics with Asymmetric, High-Spin Black Hole Coalescences鈥� was published Oct. 28 in The Astrophysical Journal Letters.聽

Other papers from GWTC-0.4 mentioned above:

摆1闭听GW231123: a Binary Black Hole Merger with Total Mass 190-265 M鈯�,听

摆2闭听Compact Binary Coalescence Sensitivity Estimates with Injection Campaigns during the LIGO-Virgo-KAGRA Collaborations鈥� Fourth Observing Run,听 (in press with PRD)

摆3闭听GWTC-4.0: Population Properties of Merging Compact Binaries,听

摆4闭听GWTC-4.0: Updating the Gravitational-Wave Transient Catalog with Observations from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run,听

摆5闭听GWTC-4.0: Methods for Identifying and Characterizing Gravitational-wave Transients,听

摆6闭听GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog,听

Contact:
Lyuba Encheva,
Communications and Events Coordinator
Canadian Institute for Theoretical Astrophysics
Email: communication@cita.utoronto.ca