Astronomy

GW231123: LIGO-Virgo-KAGRA detect most massive black hole merger to date

By ScientifiCult 14 July 2025

GW231123: LIGO-Virgo-KAGRA detect most massive black hole merger to date

Gravitational waves from massive black holes challenge current astrophysical models

The LIGO-Virgo-KAGRA (LVK) Collaboration has detected the merger of the most massive black holes ever observed with gravitational waves using the US National Science Foundation (NSF)-funded LIGO observatories. The powerful merger produced a final black hole approximately 225 times the mass of our Sun. The signal, designated GW231123, was detected during the fourth observing run of the LVK network on November 23, 2023.

The two black holes that merged were approximately 103 and 137 times the mass of the Sun. In addition to their high masses they are also rapidly spinning, making this a uniquely challenging signal to interpret and suggesting the possibility of a complex formation history.

“The discovery of such a massive and highly spinning system presents a challenge not only to our data analysis techniques – says Ed Porter, researcher at the Astroparticle and Cosmology laboratory (APC) of CNRS in Paris – but will have a major effect on the theoretical studies of black hole formation channels and waveform modelling for many years to come. Actually, current models of stellar evolution do not allow the existence of such massive black holes, which could possibly have formed through previous mergers of smaller black holes.”

Approximately 100 black-hole mergers have previously been observed through gravitational waves, analysed and shared with the wider scientific community. Until now the most massive binary was the source of GW190521, with a much smaller total mass of “only” 140 times that of the sun.

Before now, the most massive black hole merger—produced by an event that took place in 2021 called GW190521—had a total mass of 140 times that of the Sun.

In the more recent GW231123 event, the 225-solar-mass black hole was created by the coalescence of black holes each approximately 100 and 140 times the mass of the Sun.

In addition to their high masses, the black holes are also rapidly spinning.

“This is the most massive black hole binary we’ve observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation,” says Mark Hannam of Cardiff University and a member of the LVK Collaboration. “Black holes this massive are forbidden through standard stellar evolution models. One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes.”

Dave Reitze, the executive director of LIGO at Caltech, says, “This observation once again demonstrates how gravitational waves are uniquely revealing the fundamental and exotic nature of black holes throughout the universe.”

A record-breaking system

The high mass and extremely rapid spinning of the black holes in GW231123 push the limits of both gravitational-wave detection technology and current theoretical models. Extracting accurate information from the signal required the use of models that account for the intricate dynamics of highly spinning black holes.

“The black holes appear to be spinning very rapidly—near the limit allowed by Einstein’s theory of general relativity,” explains Charlie Hoy of the University of Portsmouth and a member of the LVK. “That makes the signal difficult to model and interpret. It’s an excellent case study for pushing forward the development of our theoretical tools.”

Researchers are continuing to refine their analysis and improve the models used to interpret such extreme events. “It will take years for the community to fully unravel this intricate signal pattern and all its implications,” says Gregorio Carullo of the University of Birmingham and a member of the LVK. “Despite the most likely explanation remaining a black hole merger, more complex scenarios could be the key to deciphering its unexpected features. Exciting times ahead!”

Probing the limits of gravitational-wave astronomy

The high mass and extremely rapid spinning of the black holes in GW231123 pushes the limits of both gravitational-wave detection technology and current theoretical models. Extracting accurate information from the signal required the use of theoretical models that account for the complex dynamics of highly spinning black holes.

“This event pushes our instrumentation and data-analysis capabilities to the edge of what’s currently possible,” says Dr. Sophie Bini, a postdoctoral researcher at Caltech, previously at the University of Trento. “It’s a powerful example of how much we can learn from gravitational-wave astronomy—and how much more there is to uncover.”

Gravitational-wave detectors such as LIGO in the United States, Virgo in Italy, and KAGRA in Japan are designed to measure minute distortions in spacetime caused by violent cosmic events like black hole mergers. The fourth observing run began in May 2023 and observations from the first half of the run (up to January 2024) will be published later in the summer.

“With the longest continuous observation to date and enhanced sensitivity, the LIGO-Virgo-KAGRA fourth observing campaign is delivering invaluable new insights into our understanding of the universe –says Viola Sordini, researcher at the Institute of Physics of the 2 Infinities (IP2I) of CNRS in Lyon and deputy spokesperson of the Virgo Collaboration – This exciting discovery opens a new season of results, with many more expected throughout the summer and a continued stream of findings anticipated over the next two years. Publications are followed by release of the data, in support of the broader scientific community and open science”

GW231123 will be presented at the 24th International Conference on General Relativity and Gravitation (GR24) and the 16th Edoardo Amaldi Conference on Gravitational Waves, held jointly as the GR-Amaldi meeting in Glasgow, UK, from July 14-18 2025.

LIGO, the Laser Interferometer Gravitational-wave Observatory, made history in 2015 when it made the first-ever direct detection of gravitational waves, ripples in space-time. In that case, the waves emanated from a black hole merger that resulted in a final black hole 62 times the mass of our Sun. The signal was detected jointly by the twin detectors of LIGO, one located in Livingston, Louisiana, and the other in Hanford, Washington.

Since then, the LIGO team has teamed up with partners at the Virgo detector in Italy and KAGRA (Kamioka Gravitational Wave Detector) in Japan to form the LVK Collaboration. These detectors have collectively observed more than 200 black hole mergers in their fourth run, and about 300 in total since the start of the first run in 2015.

The LIGO-Virgo-KAGRA Collaboration

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 eQort through the LIGO Scientiﬁc Collaboration, which includes the GEO Collaboration. Additional partners are listed at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 1.000 members from 175 institutions in 20 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 National Institute of Nuclear Physics (INFN) in Italy, the National Institute of Subatomic Physics (Nikhef) in the Netherlands, The Research Foundation – Flanders (FWO) e the Belgian Fund for Scientific Research (F.R.S.–FNRS). A list of the Virgo Collaboration groups can be found at: https://www.virgo-gw.eu/about/scientiﬁc-collaboration/. More information is available on the Virgo website at https://www.virgo-gw.eu.

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 http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.

Press release from EGO and California Institute of Technology

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Astronomy

Webb discovers methane, carbon dioxide in atmosphere of K2-18 b

By ScientifiCult 11 September 2023

Webb discovers methane, carbon dioxide in atmosphere of K2-18 b

A new investigation by an international team of astronomers using data from the NASA/ESA/CSA James Webb Space Telescope into K2-18 b, an exoplanet 8.6 times as massive as Earth, has revealed the presence of carbon-bearing molecules including methane and carbon dioxide. The discovery adds to recent studies suggesting that K2-18 b could be a Hycean exoplanet, one which has the potential to possess a hydrogen-rich atmosphere and a water ocean-covered surface.

methane carbon dioxide K2-18 b — This artist’s concept shows what exoplanet K2-18 b could look like based on science data. K2-18 b, an exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth. A new investigation with the NASA/ESA/CSA James Webb Space Telescope into K2-18 b has revealed the presence of carbon-bearing molecules including methane and carbon dioxide. The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be an ocean underneath a hydrogen-rich atmosphere in K2-18 b.
Credit:
NASA, CSA, ESA, J. Olmstead (STScI), N. Madhusudhan (Cambridge University)

The first insight into the atmospheric properties of this habitable-zone exoplanet came from observations with the NASA/ESA Hubble Space Telescope, which prompted further studies that have since changed our understanding of the system.

K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth in the constellation Leo. Exoplanets such as K2-18 b, which have sizes between those of Earth and Neptune, are unlike anything in our Solar System. This lack of equivalent nearby planets means that these ‘sub-Neptunes’ are poorly understood, and the nature of their atmospheres is a matter of active debate among astronomers. Exoplanets such as K2-18 b, which have sizes between those of Earth and Neptune, are unlike anything in our Solar System. This lack of analogous nearby planets means that these ‘sub-Neptunes’ are poorly understood and the nature of their atmospheres is a matter of active debate between astronomers. The suggestion that the sub-Neptune K2-18 b could be a Hycean exoplanet is intriguing, as some astronomers believe that these worlds are promising environments to search for evidence for life on exoplanets.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,”

explained Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results.

“Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations.“

The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be an ocean underneath a hydrogen-rich atmosphere on K2-18 b. These initial Webb observations also provided a possible detection of a molecule called dimethyl sulphide (DMS). On Earth, this is only produced by life. The bulk of the DMS in Earth’s atmosphere is emitted from phytoplankton in marine environments.

The inference of DMS is less robust and requires further validation.

“Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” explained Madhusudhan.

While K2-18 b lies in the habitable zone and is now known to harbour carbon-bearing molecules, this does not necessarily mean that the planet can support life. The planet’s large size — with a radius 2.6 times the radius of Earth — means that the planet’s interior likely contains a large mantle of high-pressure ice, like Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface. Hycean worlds are predicted to have oceans of water. However, it is also possible that the ocean is too hot to be habitable or be liquid.

“Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,” explained team member Subhajit Sarkar of Cardiff University. “We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere.”

Characterising the atmospheres of exoplanets like K2-18 b — meaning identifying their gases and physical conditions — is a very active area in astronomy. However, these planets are outshone — literally — by the glare of their much larger parent stars, which makes exploring exoplanet atmospheres particularly challenging.

The team sidestepped this challenge by analysing light from K2-18 b’s parent star as it passed through the exoplanet’s atmosphere. K2-18 b is a transiting exoplanet, meaning that we can detect a drop in brightness as it passes across the face of its host star. This is how the exoplanet was first discovered. This means that during transits a tiny fraction of starlight will pass through the exoplanet’s atmosphere before reaching telescopes like Webb. The starlight’s passage through the exoplanet atmosphere leaves traces that astronomers can piece together to determine the gases of the exoplanet’s atmosphere.

“This result was only possible because of the extended wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits,” continued Madhusudhan. “For comparison, one transit observation with Webb provided comparable precision to eight observations with Hubble conducted over a few years and in a relatively narrow wavelength range.”

“These results are the product of just two observations of K2-18 b, with many more on the way,” explained team member Savvas Constantinou of the University of Cambridge. “This means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

The team now intends to conduct follow-up research with the telescope’s Mid-InfraRed Instrument (MIRI) spectrograph that they hope will further validate their findings and provide new insights into the environmental conditions on K2-18 b.

“Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the Universe,” concluded Madhusudhan. “Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest.”

Spectrum of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph), displays an abundance of methane and carbon dioxide in the exoplanet’s atmosphere, as well as a possible detection of a molecule called dimethyl sulfide (DMS). The detection of methane and carbon dioxide, and shortage of ammonia, are consistent with the presence of an ocean underneath a hydrogen-rich atmosphere in K2-18 b. K2-18 b, 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth.
Credit:
NASA, CSA, ESA, J. Olmstead (STScI), N. Madhusudhan (Cambridge University)

The team’s results are accepted for publication in The Astrophysical Journal Letters.

Notes

[1] The Habitable Zone is the region around a star where the conditions could potentially be suitable to sustain life on a planet within this region, for example allowing the presence of liquid water on its surface.

Press release from ESA Webb

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Astronomy

ALMA finds possible sign of neutron star in supernova 1987A

By ScientifiCult 2 August 2020

ALMA finds possible sign of neutron star in supernova 1987A

Two teams of astronomers have made a compelling case in the 33-year-old mystery surrounding Supernova 1987A. Based on observations of the Atacama Large Millimeter/submillimeter Array (ALMA) and a theoretical follow-up study, the scientists provide new insight for the argument that a neutron star is hiding deep inside the remains of the exploded star. This would be the youngest neutron star known to date.

Ever since astronomers witnessed one of the brightest explosions of a star in the night sky, creating Supernova 1987A (SN 1987A), they have been searching for a compact object that should have formed in the leftovers from the blast.

Because particles known as neutrinos were detected on Earth on the day of the explosion (23 February 1987), astronomers expected that a neutron star had formed in the collapsed center of the star. But when scientists could not find any evidence for that star, they started to wonder whether it subsequently collapsed into a black hole instead. For decades the scientific community has been eagerly awaiting a signal from this object that has been hiding behind a very thick cloud of dust.

The “blob”

Recently, observations from the ALMA radio telescope provided the first indication of the missing neutron star after the explosion. Extremely high-resolution images revealed a hot “blob” in the dusty core of SN 1987A, which is brighter than its surroundings and matches the suspected location of the neutron star.

“We were very surprised to see this warm blob made by a thick cloud of dust in the supernova remnant,” said Mikako Matsuura from Cardiff University and a member of the team that found the blob with ALMA. “There has to be something in the cloud that has heated up the dust and which makes it shine. That’s why we suggested that there is a neutron star hiding inside the dust cloud.”

Even though Matsuura and her team were excited about this result, they wondered about the brightness of the blob. “We thought that the neutron star might be too bright to exist, but then Dany Page and his team published a study that indicated that the neutron star can indeed be this bright because it is so very young,” said Matsuura.

Dany Page is an astrophysicist at the National Autonomous University of Mexico, who has been studying SN 1987A from the start. “I was halfway through my PhD when the supernova happened,” he said, “it was one of the biggest events in my life that made me change the course of my career to try to solve this mystery. It was like a modern holy grail.”

The theoretical study by Page and his team, published today in The Astrophysical Journal, strongly supports the suggestion made by the ALMA team that a neutron star is powering the dust blob. “In spite of the supreme complexity of a supernova explosion and the extreme conditions reigning in the interior of a neutron star, the detection of a warm blob of dust is a confirmation of several predictions,” Page explained.

These predictions were the location and the temperature of the neutron star. According to supernova computer models, the explosion has “kicked away” the neutron star from its birthplace with a speed of hundreds of kilometers per second (tens of times faster than the fastest rocket). The blob is exactly at the place where astronomers think the neutron star would be today. And the temperature of the neutron star, which was predicted to be around 5 million degrees Celsius, provides enough energy to explain the brightness of the blob.

Not a pulsar or a black hole

Contrary to common expectations, the neutron star is likely not a pulsar. “A pulsar’s power depends on how fast it spins and on its magnetic field strength, both of which would need to have very finely tuned values to match the observations,” said Page, “while the thermal energy emitted by the hot surface of the young neutron star naturally fits the data.”

“The neutron star behaves exactly like we expected,” added James Lattimer of Stony Brook University in New York, and a member of Page’s research team. Lattimer has also followed SN 1987A closely, having published prior to SN 1987A predictions of a supernova’s neutrino signal that subsequently matched the observations. “Those neutrinos suggested that a black hole never formed, and moreover it seems difficult for a black hole to explain the observed brightness of the blob. We compared all possibilities and concluded that a hot neutron star is the most likely explanation.”

This neutron star is a 25 km wide, extremely hot ball of ultra-dense matter. A teaspoon of its material would weigh more than all the buildings within New York City combined. Because it can only be 33 years old, it would be the youngest neutron star ever found. The second youngest neutron star that we know of is located in the supernova remnant Cassiopeia A and is 330 years old.

Only a direct picture of the neutron star would give definite proof that it exists, but for that astronomers may need to wait a few more decades until the dust and gas in the supernova remnant become more transparent.

Detailed ALMA images

This colorful, multiwavelength image of the intricate remains of Supernova 1987A is produced with data from three different observatories. The red color shows dust and cold gas in the center of the supernova remnant, taken at radio wavelengths with ALMA. The green and blue hues reveal where the expanding shock wave from the exploded star is colliding with a ring of material around the supernova. The green represents the glow of visible light, captured by NASA’s Hubble Space Telescope. The blue color reveals the hottest gas and is based on data from NASA’s Chandra X-ray Observatory. The ring was initially made to glow by the flash of light from the original explosion. Over subsequent years the ring material has brightened considerably as the explosion’s shock wave slams into it. Credits: ALMA (ESO/NAOJ/NRAO), P. Cigan and R. Indebetouw; NRAO/AUI/NSF, B. Saxton; NASA/ESA

Even though many telescopes have made images of SN 1987A, none of them have been able to observe its core with such high precision as ALMA. Earlier (3-D) observations with ALMA already showed the types of molecules found in the supernova remnant and confirmed that it produced massive amounts of dust.

“This discovery builds upon years of ALMA observations, showing the core of the supernova in more and more detail thanks to the continuing improvements to the telescope and data processing,” said Remy Indebetouw of the National Radio Astronomy Observatory and the University of Virginia, who has been a part of the ALMA imaging team.

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This research is presented in two papers:

ALMA observation of the “blob”: “High Angular Resolution ALMA Images of Dust and Molecules in the SN 1987A Ejecta”, by P. Cigan et al., The Astrophysical Journal. https://doi.org/10.3847/1538-4357/ab4b46

Theoretical study favoring a neutron star: “NS 1987A in SN 1987A”, by D. Page et al., The Astrophysical Journal. https://doi.org/10.3847/1538-4357/ab93c2

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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Press release on Supernova 1987A from the National Radio Astronomy Observatory

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