Testing Einstein’s theory of relativity with the clearest gravitational-wave signal yet

Visualization of a binary black hole ringdown consistent with the gravitational-wave event GW250114.The gravitational waves are separated into two modes of the ringing remnant black hole, identified in the observation: the fundamental mode (green) and its first overtone (red). It also shows a predicted third tone (yellow) that the data places limits on. Visualization performed at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), based on a numerical relativity simulation of the Simulating Extreme Spacetimes (SXS) Project. Credit: H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), K. Mitman (Cornell University)

Ringing Black Hole Animation (GW250114)
Credit: H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), K. Mitman (Cornell University)

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To the point:

• Relativity put to the test: Relativity put to the test: A LIGO-Virgo-KAGRA team has conducted some of the most precise tests of Einstein’s theory of general relativity. The results were published in Physical Review Letters today.


• Einstein holds fast: In all tests, the observations match the theory’s predictions. In some cases, the tests based on this signal alone are two to three times more stringent than those obtained by combining data from dozens of other signals.


• The clearest signal: The team used data from GW250114, the strongest gravitational-wave signal ever detected from the merger of two black holes.


• Like a bell: For the first time, detailed analyses of the complete signal and the ringdown phase, which occurs shortly after the merger, have identified or constrained three gravitational-wave tones.

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An international team, with key contributions from AEI researchers, identified three gravitational-wave tones in GW250114 for the first time and conducted the most stringent tests of general relativity.

Relativity put to the test

A year ago, almost to the day, the LIGO-Virgo-KAGRA collaboration observed by far the clearest gravitational-wave signal seen to date. GW250114 came from a coalescence of black holes with masses between 30 to 40 times that of our Sun about 1.3 billion light-years away.

“This signal has already proven to be a great boon for a test of the nature of black holes and of Hawking’s area law,” says Alessandra Buonanno, director of the Astrophysical and Cosmological Relativity department at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in the Potsdam Science Park. “Now we have gone one step further and published some of the most stringent limits on deviations from Einstein’s theory of general relativity using GW250114.”

Additional analysis of the GW250114 data was published today in Physical Review Letters. The writing team included several AEI members: Alessandra Buonanno, who served as chair, and Lorenzo Pompili, Elisa Maggio, and Elise Sänger, who conducted several of the analyses reported in the publication.

Because GW250114 was observed so clearly, it can be compared in much greater detail to predictions from Einstein’s theory of relativity than other signals. This makes it possible to test whether general relativity holds true in the extreme conditions of a black hole coalescence, where strong gravitational fields meet rapidly changing dynamics. Any deviations from the predictions of general relativity could hint at new physics beyond Einstein’s theory.

Like a struck bell

The international research team obtained some of the key results using a method known as black hole spectroscopy. For this, the team focused on the ringdown of the GW250114 signal – the phase when the black hole settles into its final state right after the merger – and the characteristic spectrum of gravitational-wave modes, or tones, emitted during this phase. These tones resemble the sounds a bell makes when struck: Each tone is described by two numbers: its frequency and the rate at which it is fading. Measuring the spectrum of the tones and their fading times is called black hole spectroscopy.

For the first time, a triad of gravitational-wave tones

For the first time, researchers at the AEI in Potsdam found a third tone in the signal’s ringdown phase using a new data analysis tool they developed.

“Our analysis tool, originally proposed in 2018, takes into account the complete black-hole coalescence and makes no prior assumptions about the tones emitted during the ringdown phase,” explains Elisa Maggio, a former Marie Curie Fellow in the Astrophysical and Cosmological Relativity department and now an INFN Researcher in Rome, Italy. Maggio and Pompili collaborated on developing the most recent version of the tool and conducting the analysis. “By incorporating information from the entire signal, we constrained a higher-pitched tone at approximately twice the fundamental frequency for the first time, once again matching theoretical predictions.”

Together, the two tests – one looking at the ringdown alone and the other considering the full signal – complement each other. Once again, they empirically vindicate the rotating black hole solution discovered in 1963 by Roy Kerr.

One signal beats dozens of others

The research team also examined an earlier phase of the clearly observed black hole coalescence when the two black holes were orbiting each other more slowly.

“We used a flexible, theory-independent method developed earlier at the AEI to determine how much the gravitational-wave signal deviated from the predictions of general relativity early in the coalescence,” says Elise Sänger, a PhD student in the Astrophysical and Cosmological Relativity department who conducted the analysis. “Remarkably, using data from this one clearly observed signal alone allows us to set some of the most stringent constraints on possible deviations from general relativity.”

The constraints derived using the AEI-developed model are two to three times more stringent than those obtained by combining data from dozens of signals in the latest fourth Gravitational-Wave Transient Catalogue (GWTC-4.0).

Only the beginning

“These results demonstrate the great scientific value of accurate waveform models and ophisticated data analysis techniques,” says Alessandra Buonanno. “But this is only the beginning. Future observing runs will allow us to detect signals like GW250114 more frequently and more clearly. Each one will open new avenues for testing Einstein’s theory and searching for new physics.”

Source: Max Planck Institute for Gravitational Physics/News

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Media contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel: +49 331 567-7303
elke.mueller@aei.mpg.de

Science contacts:

Prof. Dr. Alessandra Buonanno
Director | LSC
Principal Investigator
Tel: +49 331 567-7220
Fax: +49 331 567-7298
alessandra.buonanno@aei.mpg.de
Homepage of Alessandra Buonanno

Dr. Elisa Maggio
INFN Researcher
elisa.maggio@aei.mpg.de
Istituto Nazionale di Fisica Nucleare, Rome

Dr. Lorenzo Pompili
Research Fellow
Lorenzo.Pompili@nottingham.ac.uk
University of Nottingham, School of Mathematical Sciences

Elise Sänger
PhD Student
elise.saenger@aei.mpg.de

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Publication

The LIGO Scientific Collaboration; the Virgo Collaboration; the KAGRA Collaboration; Abac, A.; Abouelfettouh, I.; Acernese, F.; Ackley, K.; Adhicary, S.; Adhikari, D.; Adhikari, N. et al.

Black Hole Spectroscopy and Tests of General Relativity with GW250114. Physical Review Letters 136, 041403 (2026)

MPG.PuRe - DOI - pre-print - publisher-version

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Further information

LIGO Science Summary
of the publication “Black hole spectroscopy and tests of general relativity with GW250114”

MPG.PuRe - DOI - pre-print - publisher-version

more

Ten years of gravitational-wave astronomy and the clearest signal yet

September 10, 2025
The LIGO-Virgo-KAGRA collaboration and the Max Planck Institute for Gravitational Physics celebrate the anniversary and present new, exciting results.

more

LIGO-Virgo-KAGRA Collaboration successfully wraps up its fourth Observing Run

November 19, 2025
Further exciting results anticipated from O4’s remaining parts

more

Doubling the gravitational-wave transient catalogue

August 26, 2025

LIGO-Virgo-KAGRA researchers at the Max Planck Institute for Gravitational Physics and at Leibniz University Hannover make significant contributions to detect and analyze new gravitational-wave candidates

more

Brito, R.; Buonanno, A.; Raymond, V.
Black-hole Spectroscopy by Making Full Use of Gravitational-Wave Modeling. Physical Review D 98 (8), 084038 (2018)

MPG.PuRe - DOI - pre-print

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An old puzzle solved: astronomers discover the world is flat

The average distribution of dark matter for a large number of computer simulations, each of which was required to form a Milky Way and an Andromeda Nebula (the two bright blobs at the centre) with the observed position and velocity, and also to match the observed velocity at the position of 31 nearby galaxies (cyan dots). The box size is 20 times the Milky Way-Andromeda separation with a depth of one-half this separation. Colour represents the amount of dark matter at each point, while arrows show its velocity relative to a uniformly expanding universe. The left image is looking down onto the Local Mass Sheet, while the right one views it from the side. Notice that velocities relative to a uniform Hubble flow are small in both panels in the region occupied by the cyan dots, implying that these galaxies appear to match Hubble’s Law almost perfectly in the simulated universes. © MPA

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A pan-European group of astronomers has used newly developed computer technology to solve a 100 year-old puzzle. While most galaxies in our neighborhood move away from us almost as expected for an unperturbed cosmic expansion, our nearest giant neighbour is approaching at high speed. Systematic numerical experimentation demonstrates this rapid approach is due to massive dark matter haloes surrounding both Andromeda and our own Milky Way, but this mass does not slow down somewhat more distant galaxies because its effects are counteracted by more distant dark matter which lies in a vast flattened sheet out to distances well beyond the neighboring galaxies considered.

Why is the Andromeda Nebula heading straight for us, while other nearby galaxies are receding?

It is nearly a century since the American astronomer Edwin Hubble discovered the expansion of the Universe. Distant galaxies similar to our own Milky Way move away from us at speeds that increase in proportion to their distance, reflecting the origin of the Universe in a Big Bang, an enormous explosion 14 billion years ago. Hubble already knew, however, that this is not true for our nearest giant neighbour, the Andromeda Nebula, which is 2.5 million light-years away and coming towards us at 100 kilometers per second. In 1959, two European astronomers, Franz Kahn and Lodewijk Woltjer, calculated that in order for the gravity of the two galaxies to have reversed the initial expansion, their total mass must be more than 1000 billion times the mass of the Sun – much more than the mass of all their stars put together. This was the first detection of unseen Dark Matter around our Milky Way and its neighbour.

In the 1970s and 1980s, accurate distances began to be measured for somewhat more distant galaxies. It became clear that not only are they are mostly moving away from us but that their speeds are close to those predicted by the overall cosmic expansion – starting in a “Big Bang” 14 billion years ago. Studies of galaxies at distances from 1.5 to 4 times the Milky Way-Andromeda separation found the deviations to be actually quite small – the total amount of matter required to account for these deviations out to the most distant galaxy cannot be larger than that already needed to explain the approach speed of the Milky Way and Andromeda. However, there are several other large galaxies in this region, which should contribute additional mass. Why then does the cosmic expansion around us appear so weakly perturbed?

A pan-European group of astronomers has recently used newly developed computer technology to find the solution to this puzzle. They set the machine the following task: Find representative regions of the early Universe with small deviations from uniformity that are statistically similar to the Cosmic Microwave Background, but that evolve to produce galaxies similar to the Milky Way and Andromeda, with the appropriate positions and velocities. At the same time, other nearby galaxies should show motions and positions matching those of observed nearby galaxies.

Apparently, the puzzle was not hard for the computer: it was able to find hundreds of examples satisfying all the given conditions. The average mass distribution for a large number of these is shown in the figure. In the region containing the local galaxies, motions relative to a uniform expansion are indeed small – the Hubble flow is almost unperturbed – while at larger distances material is actually moving away from the Milky Way faster than the Hubble flow.

Max Planck Institute for Astrophysics How the computer solved the puzzle can be seen in the right image of the figure, which shows a view of the same box rotated by 90 degrees. The mass is concentrated to a flattened sheet extending well beyond the region occupied by the local galaxies considered. All the galaxies are inside the sheet and even at larger distances most known galaxies are still found in a flattened distribution known as the Local Supercluster. The computer has inferred this larger structure even though it was not told about its existence. The large low-density regions above and below the sheet are also seen in the galaxy distribution and are known as the Local Voids. However, the large velocities predicted there are not observable, because in the real universe there are no galaxies there to be measured.

Thus, there are two reasons why the local Hubble flow seems so weakly perturbed despite the large combined mass of the Milky Way and Andromeda. Mass at larger distances is counteracting the gravity of the central galaxies by pulling material outwards. In addition, there are no galaxies where the predicted infall effects are large, so inflow onto the Local Sheet is hidden.

The solution to the puzzle is that the total mass distribution in our environment is at least as sheet-like as the distribution of galaxies. The world around our Local Group of galaxies is indeed flat out to distances of tens of millions of light-years.

Source: Max Planck Institute for Astrophysics/Press Releaes

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Author:

Simon White
Emeritus Director
Tel: 2211
Tel: +49 170 248 1178
swhite@mpa-garching.mpg.de

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Original publication

E. Wempe et al. The mass distribution in and around the Local Group
Nature Astronomy, 27 January 2026

Source

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Weitere Informationen

L’anomalie d’Andromède résolue : une feuille cosmique explique son mouvement et l’expansion locale
CNRS Press Release
(in French)

Een ‘platte’ omgeving van de Melkweg verklaart de beweging van nabije sterrenstelsels
Dutch press release

Gammal gåta löst: astronomer upptäcker att vårt kosmiska närområde är platt
University Stockholm press release

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NuSTAR Observes an Unusual Black Hole

An artist's illustration of hot gas from a low-mass companion star flowing into an accretion disk around the black hole. Image credit: ESO/L. Calçada. - Download Image

During the past week, NuSTAR performed multiple observations of the black hole X-ray binary (BHXB) GS 1354−64, including one observation coordinated with the JAXA/ESA/NASA mission XRISM. GS 1354−64 is unusual in several respects. Discovered in 1987 during an outburst, it has since undergone two additional outbursts (1997 and 2015), yet in both cases it “failed” to complete the canonical hard-to-soft X-ray spectral transition, observed in outbursts by other BHXB, before fading back into quiescence. In the current episode, however, the most recent NuSTAR observation indicates that the source is now transitioning toward the soft X-ray state. Optical studies of the companion star (BW Cir) have placed the system at a distance of ~27 kpc, which—if correct—would make it among the most distant known Galactic BHXBs. A preliminary analysis of the latest NuSTAR data suggests that, at this distance, the transition would be occurring at an X-ray luminosity of roughly 70% of the Eddington limit (the luminosity at which radiation pressure balances gravity for accreting material). This is striking because BHXBs typically undergo the hard-to-soft transition at ≲10% of the Eddington limit. The NuSTAR dataset will therefore be used to test several possibilities: is the source substantially closer than we thought it was (e.g., 8–10 kpc and thus less luminous), significantly more massive than currently assumed (and thus would have a higher Eddington limit), or otherwise genuinely anomalous? In addition, the combination of the high spectral resolution and broad X-ray sensitivity from the joint NuSTAR+XRISM observation will enable tighter constraints on the binary system’s spin and inclination.

Author: Oluwashina Adegoke (Postdoctoral Scholar, Caltech)

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/Weekly Highlights)

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Binary dance: a white dwarf system explains mysterious radio pulses across the Milky Way

Artistic impression of the red dwarf and white dwarf interaction of the long-period transient GPM J1839-10. Credits: D. Futselaar/Horvath, Rea, Hurley-Walker et al. 2026

The white and red spheres are the white dwarf and M-dwarf. The arrow represents the white dwarf’s rotating magnetic moment. The yellow cone is the radio beam whose brightness depends on the alignment of the white dwarf’s magnetic moment with the M-dwarf. Below is the radio flux density detected on Earth. Animation from interactive by Csanad Horvath.

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Investigating the nature of long-period transients, a newly discovered class of radio transients, can widen our knowledge on how plasmas and magnetic fields interact in extreme environments.

An international team observed a long-period transient non stop for 2 days in a study published in Nature Astronomy.

Over the past four years, a new class of signals from the Universe has captured astronomers’ attention. These events originate from galactic objects known as long-period transients (LPTs), whose nature remains unknown. They appear as repeating bright radio pulses with unusually long periods. So far, around 12 such sources have been discovered, but their origin and the mechanisms that generate their emission are still unclear. A new study published in Nature Astronomy, led by ICRAR PhD student Csanad Horvath, investigates the longest-lived LPT known.

GPM J1839-10 is the name of the longest-known LPT, with a 21- min period, observed in the Milky Way. It has now been demonstrated to be a binary system hosting a white dwarf. This system consists of a spinning white dwarf (a stellar remnant) and a red dwarf (a star smaller than the Sun). It adds scientific evidence to two previous studies that proposed other LPTs as the same type of binary systems, showing that most LPTs may share a similar origin. The exact physics behind the bright radio pulses of these sources has now been related with the interaction between the white dwarf pulsar magnetic field and the wind of the companion star.

“This work demonstrates a novel way of shedding light on the nature of LPTs, a field that started only 3 years ago and has been revealed to be key to understanding the radio transient sky”, says Nanda Rea, from the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC), co-author of the study.

These findings may represent the first steps toward understanding the true nature of all LPTs and, consequently, revising our knowledge of white dwarf and red dwarf binaries. “Radio emission produced by white dwarf binaries might be more prevalent and diverse than previously thought,” suggests Rea.

To carry out the study, the international team, composed of six researchers from astronomy research institutes in Australia and Spain, conducted a continuous 40-hour observation of GPM J1839−10. This required three radio telescopes operating sequentially at different locations around the world: the MeerKAT telescope in South Africa, CSIRO’s ASKAP radio telescope on Wajarri Yamaji Country in Australia, and the Karl G. Jansky Very Large Array (VLA) in the United States of America. “Each telescope handed the source to the next as the Earth rotated to keep the source in view,” explains Csanad Horvath, a PhD student at the Curtin University node of ICRAR, who led the work and spent a month at ICE-CSIC to finalize the analysis.

This allowed the team to record the signal pattern with high precision for subsequent analysis. They discovered that the radio pulses arrive in groups of 4 or 5 and that they come in pairs separated by two hours. A pattern that repeats every 9 hours, suggesting orbital motion with such period happens within the source system.

Using a theoretical model based on the same geometric framework proposed for white dwarf pulsars, the team accurately reproduced the intermittent emission and double-pulse structure. This strongly supports the interpretation that the LPT is a white dwarf–red dwarf binary system, and allowed to measure the system characteristics as the orbit, the inclination in our line of sight and star masses. In this scenario, radio pulses are produced whenever the magnetic axis of the spinning white dwarf, the imaginary line connecting its two magnetic poles, intersects the stellar wind of its companion, generating a bright radio signal. In every orbit the bright radio pulses can be seen twice, with 4-5 pulses every time.

Beyond revealing the likely nature of one known LPT, the study also provides a framework to investigate many more such objects. The model applied to the growing population of LPTs and other known white dwarf binary pulsars have shown the connection between these apparently different classes as well as shed light on the evolution of magnetic properties of white dwarf and red dwarf binaries.

Source: International Center for Radio Astronomy Research (ICRAR)/News

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Publication

Horváth, Rea, Hurley-Walker, McSweeney, Perley, & Lenc (2026), ‘A binary model of long-period radio transients and white dwarf pulsars‘, Nature Astronomy. DOI: 10.1038/s41550-025-02760-y.

Adapted from a media release by Institute of Space Studies of Catalonia (IEEC)

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NASA Webb Pushes Boundaries of Observable Universe Closer to Big Bang

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COSMOS Field MoM-z14 Galaxy (NIRCam Image)

NASA’s James Webb Space Telescope shows galaxy MoM-z14 as it appeared in the distant past, only 280 million years after the universe began in the big bang. Credit Image: NASA, ESA, CSA, STScI, Rohan Naidu (MIT); Image Processing: Joseph DePasquale (STScI)

COSMOS Field MoM-z14 Galaxy (NIRCam Compass Image)

NASA’s James Webb Space Telescope shows galaxy MoM-z14 as it appeared in the distant past, only 280 million years after the universe began in the big bang. Credit Image: NASA, ESA, CSA, STScI, Rohan Naidu (MIT); Image Processing: Joseph DePasquale (STScI

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NASA’s James Webb Space Telescope has topped itself once again, delivering on its promise to push the boundaries of the observable universe closer to cosmic dawn with the confirmation of a bright galaxy that existed 280 million years after the big bang. By now Webb has established that it will eventually surpass virtually every benchmark it sets in these early years, but the newly confirmed galaxy, MoM-z14, holds intriguing clues to the universe’s historical timeline and just how different a place the early universe was than astronomers expected.

“With Webb, we are able to see farther than humans ever have before, and it looks nothing like what we predicted, which is both challenging and exciting,” said Rohan Naidu of the Massachusetts Institute of Technology’s (MIT) Kavli Institute for Astrophysics and Space Research, lead author of a paper on galaxy MoM-z14 published in the Open Journal of Astrophysics.

Due to the expansion of the universe that is driven by dark energy, discussion of physical distances and “years ago” becomes tricky when looking this far. Using Webb’s NIRSpec (Near-Infrared Spectrograph) instrument, astronomers confirmed that MoM-z14 has a cosmological redshift of 14.44, meaning that its light has been travelling through (expanding) space, being stretched and “shifted” to longer, redder wavelengths, for about 13.5 of the universe’s estimated 13.8 billion years of existence.

“We can estimate the distance of galaxies from images, but it’s really important to follow up and confirm with more detailed spectroscopy so that we know exactly what we are seeing, and when,” said Pascal Oesch of the University of Geneva, co-principal investigator of the survey.

Intriguing Features

MoM-z14 is one of a growing group of surprisingly bright galaxies in the early universe – 100 times more than theoretical studies predicted before the launch of Webb, according to the research team.

“There is a growing chasm between theory and observation related to the early universe, which presents compelling questions to be explored going forward,” said Jacob Shen, a postdoctoral researcher at MIT and a member of the research team.

One place researchers and theorists can look for answers is the oldest population of stars in the Milky Way galaxy. A small percentage of these stars have shown high amounts of nitrogen, which is also showing up in some of Webb’s observations of early galaxies, including MoM-z14.

“We can take a page from archeology and look at these ancient stars in our own galaxy like fossils from the early universe, except in astronomy we are lucky enough to have Webb seeing so far that we also have direct information about galaxies during that time. It turns out we are seeing some of the same features, like this unusual nitrogen enrichment,” said Naidu.

With galaxy MoM-z14 existing only 280 million years after the big bang, there was not enough time for generations of stars to produce such high amounts of nitrogen in the way that astronomers would expect. One theory the researchers note is that the dense environment of the early universe resulted in supermassive stars capable of producing more nitrogen than any stars observed in the local universe.

The galaxy MoM-z14 also shows signs of clearing out the thick, primordial hydrogen fog of the early universe in the space around itself. One of the reasons Webb was originally built was to define the timeline for this “clearing” period of cosmic history, which astronomers call reionization. This is when early stars produced light of high enough energy to break through the dense hydrogen gas of the early universe and begin travelling through space, eventually making its way to Webb, and us. Galaxy MoM-z14 provides another clue for mapping out the timeline of reionization, work that was not possible until Webb lifted the veil on this era of the universe.

Legacy of Discovery Continues

Even before Webb’s launch, there were hints that something very unanticipated happened in the early universe, when NASA’s Hubble Space Telescope discovered the bright galaxy GN-z11 400 million years after the big bang. Webb confirmed the galaxy’s distance — at the time the most distant ever. From there Webb has continued to push back farther and farther in space and time, finding more surprisingly bright galaxies like GN-z11.

As Webb continues to uncover more of these unexpectedly luminous galaxies, it’s clear that the first few were not a fluke. Astronomers are eagerly anticipating that NASA’s upcoming Nancy Grace Roman Space Telescope, with its combination of high-resolution infrared imaging and extremely wide field of view, will boost the sample of these bright, compact, chemically enriched early galaxies into the thousands.

“To figure out what is going on in the early universe, we really need more information —more detailed observations with Webb, and more galaxies to see where the common features are, which Roman will be able to provide,” said Yijia Li, a graduate student at the Pennsylvania State University and a member of the research team. “It’s an incredibly exciting time, with Webb revealing the early universe like never before and showing us how much there still is to discover.”

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

Source: NASA's James Webb Space Telescope/Press Releases

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Related Links

Read more: Webb Science: Galaxies Through Time

Explore more: ViewSpace Seeing Farther: Hubble Ultra Deep Field

Video: JADES: GOODS South Fly-Through Visualization

Video: Ultra Deep Field: Looking Out into Space, Looking Back into Time

Explore more: ViewSpace Gathering Light: Hubble Ultra Deep Field

More Webb News

More Webb Images

Webb Science Themes

Webb Mission Page

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New Even Horizon Telescope Results Trace M87 Jet Back to Its Black Hole

A Hubble Space Telescope image of the giant elliptical galaxy M87 with its blowtorch-like jet. The visible part of this giant stream of particles spans around 3000 light-years. © NASA, ESA, A. Lessing (Stanford University), E. Baltz (Stanford University), M. Shara (AMNH), J. DePasquale (STScI)

At 230 GHz (bottom), data from the EHT reveal the fine structure of the ring surrounding the supermassive black. © Bottom: Saurabh et al.: “Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations”, Astronomy & Astrophysics 705 (2026), Figure 6. Upper Right: Lu, R.-S. et al.: “A ring-like accretion structure in M87 connecting its black hole and jet”. Nature 616 (2023), Figure 1

Selected sites from the 2021 EHT observing campaign, highlighting additional stations: the 12−m Kitt Peak (KP) Telescope, USA and the NOrthern Extended Millimeter Array (NOEMA), France. This introduces two critical intermediate-length baselines to the Submillimeter Telescope (SMT), USA and IRAM 30−m, Spain, providing sensitivity to emission structures close to the base of the jet. © Saurabh/MPIfR

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To the point:

• Recently published data from the Event Horizon Telescope (EHT) of the galaxy Messier 87 facilitate new insights into the direct environment of the central supermassive black hole.

• Measured differences in the radio light on different spatial scales can be explained by the presence of an as of yet undetected jet at frequencies of 230 Gigahertz at spatial scales comparable to the size of the black hole.

• The most likely location of the jet base is determined through detailed modeling.

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Observations with the Event Horizon Telescope enable researchers to localize the likely base of the central outflow in a massive galaxy

Some galaxies eject powerful streams of charged particles—jets—from their centers into space. The prominent jet of Messier 87 (M87) in the constellation Virgo is visible over distances of 3000 light-years and can be observed over the full electromagnetic spectrum. It is powered by the central engine, the supermassive black hole at the heart of the galaxy with a mass of around six billion times that of our Sun. The exact location around the black hole where the jets originate is still unknown. Using observations from the Event Horizon Telescope (EHT) from 2021, an international research team led by Saurabh (Max Planck Institute for Radio Astronomy, MPIfR), Hendrik Müller (National Radio Astronomy Observatory, NRAO) and Sebastiano von Fellenberg (formerly at MPIfR, currently at the Canadian Institute for Theoretical Astrophysics, CITA) has found first hints of the jet base in M87. The results are published in the current issue of the journal Astronomy & Astrophysics.

Observing different scales

M87*, the supermassive black hole at the center of the galaxy M87, is about 55 million light years (5 × 10²⁰ kilometers) away from Earth. In 2019, the first images of its shadow and the glowing ring of hot gas around it went around the world. In order to resolve these structures, radio telescopes around the world must be combined into a single virtual telescope such as the EHT. This technique is called Very Long Baseline Interferometry (VLBI). The images produced in this way are sensitive to emission on different scales, depending on the distances between telescopes (baselines): With long baselines of several thousand kilometers, the smallest structures—such as the luminous ring—around M87* can be depicted. Short baselines of a few hundred meters, on the other hand, reveal emission emanating from much larger spatial scales in M87 (the extended jet), but are blind to details near the black hole. Intermediate baselines of a few hundred to a few thousand kilometers are the important link. They can be used to establish a connection between the material around the black hole and the jet. Precisely these intermediate baselines enabled the research team to determine the probable position of the jet base. "This study represents an early step toward connecting theoretical ideas about jet launching with direct observations. Identifying where the jet may originate and how it connects to the black hole’s shadow, adds a key piece to the puzzle and points toward a better understanding of how the central engine operates", explains Saurabh.

The decisive difference

The researchers find hints to the base of the jet by comparing the measured radio intensity on different spatial scales: On short to intermediate baselines, the measured intensity is higher compared to that on long baselines. This indicates that what is observed with long baselines—the luminous ring of hot gas around the black hole—is not solely responsible for the detected radio emission. Instead, the current data show that part of the missing emission is captured on intermediate baselines. One possibility is the jet, which has not yet been observed at a radio frequency of 230 gigahertz (GHz) with the EHT.

EHT observations from 2017 and 2018 lacked the intermediate baselines to detect it. However, with the recently published data, Saurabh's team was able to show with numerous model calculations that part of the missing emission can be best explained by an additional compact region. From our perspective, this region is about 0.09 light-years away from M87* and associated with the base of the jet. The position of the region appears to coincide with the southern arm of a radio jet discovered at a different frequency (86 GHz) in 2018. "We have observed the inner part of the jet of M87 with global VLBI experiments for many years, with ever increasing resolution, and finally managed to resolve the black hole shadow in 2019. It is amazing to see that we are gradually moving towards combining these breakthrough observations across multiple frequencies and complete the picture of the jet launching region", says Hendrik Müller.

What’s next?

The current study shows that these interesting structures around M87* become visible at radio frequencies of 230 GHz with intermediate baselines. However, further observations with the EHT will be necessary to further constrain the morphology of the jet. These observations will then make it possible to not only deduce structures such as the jet base, but to image them. This opens up new possibilities for probing the direct environment of supermassive black holes and for testing theories of black hole physics. "Newly observed data—now being correlated and calibrated with support from MPIfR—will soon add back the Large Millimetre Telescope in Mexico. This will bring an even sharper view of the jet‑launching region within reach", says Sebastiano von Fellenberg.

Source: Max Planck for Radio Astronomy/Press Releases

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Additional Information

The following scientists affiliated to the MPIfR are coauthors of this publication: Saurabh, Sebastiano D. von Fellenberg, Michael Janssen, Thomas P. Krichbaum, Dhanya G. Nair, Walter Alef, Rebecca Azulay, Uwe Bach, Anne-Kathrin Baczko, Silke Britzen, Gregory Desvignes, Sergio A. Dzib, Ralph P. Eatough, Christian M. Fromm, Ramesh Karuppusamy, Joana A. Kramer, Michael Kramer, Jun Liu, Andrei P. Lobanov, Ru-Sen Lu, Nicholas R. MacDonald, Nicola Marchili, Karl M. Menten, Cornelia Müller, Georgios Filippos Paraschos, Alexander Plavin, Eduardo Ros, Helge Rottmann, Alan L. Roy, Tuomas Savolainen, Lijing Shao, Pablo Torne, Efthalia Traianou, Jan Wagner, Robert Wharton, Gunther Witzel, Jompoj Wongphexhauxsorn, J. Anton Zensus, and Guang-Yao Zhao.

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Contacts:

Mr. Saurabh
Tel: +49 228 525-366
saurabh@mpifr-bonn.mpg.de
Max Planck Institute for Radio Astronomy, Bonn

Dr. Hendrick Müller
Tel: +1 626 781-0043
hmuller@nrao.edu
National Radio Astronomy Observatory (NRAO), USA

Dr. Sebastiano von Fellenberg
Tel: +1 437 328-5547
sfellenberg@utoronto.ca
Canadian Institute for Theoretical Astrophysics (CITA), University of Toronto, Canada

Dr. Nina Brinkmann
Press and Public Relations
Tel: +49 228 525-399
brinkmann@mpifr-bonn.mpg.de
Max Planck Institute for Radio Astronomy, Bonn

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Original publication

Saurabh et al.
Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations
Astronomy & Astrophysics 705 (2026)
[doi.org/10.1051/0004-6361/202557022]

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Parallel Press Release

CITA Press Release
From the Canadian Institute for Theoretical Astrophysics

Saurabh/MPIfR

Animation (open in full screen)

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Images

• eht_closure_phase_triangles 1.56 MB


• m87_possible_jet_base 1.13 MB


• m87_possible_jet_base_blank 1.11 MB

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The Black Hole Meetup: EMRIs and IMRIs in the Same Active Galactic Nucleus Disk

llustration of stellar-mass black holes embedded within the accretion disk of a supermassive black hole.
Credit: Caltech/R. Hurt (IPAC)

Title:The Fate of EMRI-IMRI Pairs in Active Galactic Nucleus Accretion Disks: Hydrodynamical and Three-body Simulations

Authors: Peng Peng et al.
First Author’s Institution: Peking University
Status: Published in ApJ

Active galactic nuclei (AGNs) are the extremely luminous central regions of some galaxies, powered by gas accreting onto their supermassive black holes and often outshining the entire galaxy in which they reside. One reason they are so studied in astronomy is that they connect many pieces of physics and astronomy in one cosmic place. This is especially true for AGNs as potential gravitational wave sources. Gravitational waves are observed when two compact objects, usually black holes, orbit each other. Black holes span a massive range of masses, but they are typically categorized into one of three categories: stellar-mass black holes, or sBHs (tens to hundreds of times the mass of the Sun), intermediate-mass black holes or IMBHs (hundreds to thousands of times the mass of the Sun), and supermassive black holes or SMBHs (millions to billions of times the mass of the Sun). AGNs are special because they are among the very few places where black holes across this entire mass spectrum might be found in the same place at the same time. Not only do they host SMBHs at their centers, but their gas disks are ideal nurseries for capturing and growing sBHs and IMBHs.

Current-generation gravitational wave detectors like the LIGO–Virgo–KAGRA (LVK) network can observe stellar-mass to lite-IMBH black hole mergers. Future detectors like the Laser Interferometer Space Antenna (LISA) will be able to observe black holes in the intermediate-mass to supermassive mass range. In addition to the mass range a detector can detect, it is also valuable to know the mass ratio (usually denoted by q) that a detector might detect. Unequal-mass-ratio mergers can tell us a lot about general relativity that more equal-mass mergers cannot because the smaller object orbits the more massive one many times right before the merger, essentially providing a gravitational wave measurement of spacetime around the larger black hole. (See this video for an example of black hole orbits with a large mass ratio, and imagine the spacetime observations around the larger black hole that could be possible with orbits like that.)

One of the most promising advances in gravitational wave detection with LISA will come with the observation of extreme-mass-ratio inspirals (EMRIs), usually defined as involving a smaller black hole that is at least 10,000 times less massive than the massive black hole it orbits (though the exact mass ratio defining an EMRI is a matter of convention and may vary somewhat). In addition, LISA will be able to observe intermediate-mass-ratio inspirals (IMRIs), usually defined as when the smaller black hole is 100–10,000 times less massive than the larger one.

This article uses multiple techniques to address the question of what happens when an AGN disk hosts both an IMBH and an sBH at the same time. The authors began with a hydrodynamic simulation of an AGN gas disk around a 10⁶-solar-mass SMBH. They add a 10³-solar-mass IMBH into the gas disk. Because the IMBH orbits within a gas disk, the gas exerts a force on it, causing its orbit to shrink toward the SMBH (a process called migration). Additionally, the IMBH carves out a path through the gas. Once the IMBH carves out enough of a path in the gas, the authors add an sBH of 20 solar masses into the simulation near the IMBH.

The authors test the sBH outcome for two initial conditions of the gas disk. The first, which I will refer to as “InnerDisk,” is when gas already exists inside the IMBH’s orbit (see Figure 1). The other, which I will refer to as “NoInnerDisk,” is when the simulation begins with gas only outside the IMBHs orbit, with no gas initially between the IMBH and SMBH. In this case, gas crosses the IMBH’s gap after the simulation starts. In the InnerDisk case, the sBH initially gets pushed inward from the presence of the inner gas, but that gas steadily drains into the SMBH and is only partly refilled, so the gas’s push on the sBH weakens over time. In the NoInnerDisk case, the IMBH’s direct pull on the sBH becomes more important. The amount of gas that leaks across the IMBH orbit into the inner disk gradually settles into a steady state that is less dense than the InnerDisk case. With a weaker gas push, the sBH stays closely tied to the IMBH and migrates inward at nearly the same rate. In both setups, the IMBH carves a gap in the gas and keeps moving inward, but the presence and evolution of inner gas chiefly determine how closely the sBH can keep up.

Figure 1: Simulated gas disk around an SMBH with an implanted IMBH at 0 days (left), 10.3 days (middle), and 155 days (right). Brighter orange indicates higher gas density, and darker orange/red indicates lower gas density. An sBH was added at 100 days and is seen in the right panel. These three panels represent the “InnerDisk” scenario. Adapted from Peng et al. 2025

Once the sBH and IMBH migrate close enough to the SMBH, gravitational waves are responsible for more and more of the energy loss and orbital decay of the system compared to the gas. To account for that, once the sBH and IMBH migrate close enough to the SMBH, instead of using a hydrodynamic simulation of a gas disk, the authors switch over to a “three-body problem” solver. Because these are black holes emitting gravitational waves, regular old Newtonian mechanics is insufficient, so they add post-Newtonian terms to correct for this. Additionally, though they no longer model the gas hydrodynamically, they do include terms for a gas “force” acting on the black holes to mimic the gas disk.

Once the IMBH and sBH were in the gravitational regime, their outcomes became much more chaotic. As seen in Figure 2, a slight change in the initial phase angle of the sBH can lead to drastically different consequences for the system. In some cases, the sBH is ejected entirely. In other instances, it merges with the IMBH soon after the simulation begins. Yet in others, it first merges with the SMBH. This leads to one overall message of this article: the orbits of the IMBH and sBH tend to be regular with some gentle variation when they are farther out in the gas disk, but they become highly chaotic once they shrink into the gravitational wave regime closer to the central SMBH.

Figure 2: Post-Newtonian simulation outcomes for different values of sBH initial phase angle from 0 to 2π in increments of 0.02π, while all other initial conditions were kept fixed. Green (binary formation) represents a merger of the IMBH and sBH before reaching the SMBH; red (EMRI after ejection) represents the sBH being ejected from the system, but not before some of its orbits can be observed as an EMRI event; EMRI-IMRI (orange) represents the sBH merging with the SMBH, followed by the merger of the IMBH with the SMBH; and blue (ejection) represents the sBH being ejected from the system entirely before entering a gravitational wave EMRI regime. Credit: Peng et al. 2025

AGN disks may provide a natural setting for interactions among stellar-mass, intermediate-mass, and supermassive black holes. The authors of this article demonstrated that gas can keep an IMBH and an inner sBH migrating together until gravitational waves dominate, after which slight differences in their orbital phases can lead to a wide range of outcomes. While uncertainties remain, this study provides more evidence that LISA could identify. Until LISA flies, continued simulations like this one will help refine the spectrum of EMRIs and IMRIs we can expect to see.

Original astrobite edited by Maggie Verrico

Source: American Astronomical Society/AAS-Nova

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About the author, William Smith:

Bill is a graduate student in the astrophysics program at Vanderbilt University. He studies gravitational wave populations with a focus on how these populations can help inform cosmology as part of the LIGO Scientific Collaboration. Outside of astrophysics, he also enjoys swimming semi-competitively, music and dancing, cooking, and making the academy a better place for people to live and work.

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org

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AI Unlocks Hundreds of Cosmic Anomalies in Hubble Archive

Astrophysical Anomalies from Hubble’s Archive

Six previously undiscovered, weird and fascinating astrophysical objects are displayed in this new image from NASA’s Hubble Space Telescope. They include three lenses with arcs distorted by gravity, one galactic merger, one ring galaxy, and one galaxy that defied classification.

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A team of astronomers has employed a cutting-edge, artificial intelligence-assisted technique to uncover rare astronomical phenomena within archived data from NASA’s Hubble Space Telescope. The team analyzed nearly 100 million image cutouts from the Hubble Legacy Archive, each measuring just a few dozen pixels (7 to 8 arcseconds) on a side. They identified more than 1,300 objects with an odd appearance in just two and a half days — more than 800 of which had never been documented in scientific literature.

Most of the anomalies were galaxies undergoing mergers or interactions, which exhibit unusual morphologies or trailing, elongated streams of stars and gas. Others were gravitational lenses, where the gravity of a foreground galaxy distorts spacetime and bends light from a background galaxy into arcs or rings. Additional discoveries included galaxies with massive star-forming clumps, jellyfish-looking galaxies with gaseous “tentacles,” and edge-on planet-forming disks in our own galaxy resembling hamburgers. Remarkably, several dozen objects defied existing classification schemes entirely.

Identifying such a diverse array of rare objects within the vast and growing repository of Hubble and other telescope data presents a formidable challenge. Never in the history of astronomy has such a volume of observational data been available for analysis.

To address this challenge, researchers David O’Ryan and Pablo Gómez of ESA (the European Space Agency) developed an AI tool capable of inspecting millions of astronomical images in a fraction of the time required by human experts. Their neural network, named AnomalyMatch, was trained to detect rare and unusual objects by recognizing patterns in data — mimicking the way the human brain processes visual information.

“Archival observations from the Hubble Space Telescope now span 35 years, offering a rich dataset in which astrophysical anomalies may be hidden,” said David O’Ryan, lead author of the study published in Astronomy & Astrophysics.

Traditionally, anomalous images are discovered through manual inspection or serendipitous observation. While expert astronomers excel at identifying unusual features, the sheer volume of Hubble data makes comprehensive manual review impractical. Citizen science initiatives have helped expand the scope of data analysis, but even these efforts fall short when faced with archives as extensive as Hubble’s or those from wide-field survey telescopes like Euclid, an ESA mission with NASA contributions.

The work by O’Ryan and Gómez represents a significant advancement. By applying AnomalyMatch to the Hubble Legacy Archive, they conducted the first systematic search for astrophysical anomalies across the entire dataset. After the algorithm flagged likely candidates, the researchers manually reviewed the top-rated sources and confirmed more than 1,300 as true anomalies.

“This is a powerful demonstration of how AI can enhance the scientific return of archival datasets,” Gómez said. “The discovery of so many previously undocumented anomalies in Hubble data underscores the tool’s potential for future surveys.”

Hubble is just one of many astronomical archives poised to benefit from AI-driven analysis. Facilities such as NASA’s upcoming Nancy Grace Roman Space Telescope, a well as ESA’s Euclid and the National Science Foundation and Department of Energy’s Vera C. Rubin Observatory, will generate unprecedented volumes of data. Tools like AnomalyMatch will be essential for navigating this data deluge, enabling astronomers to uncover new and unexpected phenomena — and perhaps even objects never before seen in the universe.

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Source: NASA's Hubble Space Telescope/News

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Editor: Andrea Gianopoulos

Location: NASA Goddard Space Flight Center

Contact Media:

Claire Andreoli
NASA’s Goddard Space Flight Center
Greenbelt, Maryland
claire.andreoli@nasa.gov

Bethany Downer
ESA/Hubble
Baltimore, Maryland

Ann Jenkins and Christine Pulliam
Space Telescope Science Institute
Baltimore, Maryland

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Related Links and Documents

• Science Paper: Identifying astrophysical anomalies in 99.6 million source cutouts from the Hubble legacy archive using AnomalyMatch, PDF (47.32 MB)


• Release on ESA/Hubble Website

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NASA's Chandra Releases Deep Cut From Catalog of Cosmic Recordings

X-ray Images of Sagittarius A*
Credit: X-ray: NASA/CXC/SAO; Image Processing: NASA/CXC/SAO/N. Wolk

JPEG (289.9 kb) - Large JPEG (3.4 MB) - Tiff (74.4 MB) - More Images

A Tour of the Chandra Source Catalog - More Videos

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• The Chandra Source Catalog (CSC) is a free-to-all resource that compiles the detections made by NASA’s X-ray Observatory.


• Scientists use the CSC to combine Chandra’s X-rays with data from other missions like NASA’s James Webb and Hubble Space Telescopes.


• The latest version of the CSC was released in fall 2025 and contains over 1.3 million individual X-ray detections across the sky.


• A new image of the Galactic Center illustrates that, showing 3,300 Chandra sources in this field of view that spans just 60 light-years across.

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Like a recording artist who has had a long career, NASA’s Chandra X-ray Observatory has a “back catalog” of cosmic recordings that is impossible to replic,ate. To access these X-ray tracks, or observations, the mission has developed the ultimate compendium: the Chandra Source Catalog.

The Chandra Source Catalog contains the X-ray data detected by Chandra, the world’s premier X-ray telescope and one of NASA’s “Great Observatories,” from its launch in 1999 up to the end of 2021. [AF2.1]The latest version of the Chandra Source Catalog, known as CSC 2.1, contains over 400,000 unique compact and extended sources and over 1.3 million individual detections in X-ray light.

Within the Chandra Source Catalog, there is a wealth of information gleaned from the Chandra observations — from precise positions on the sky to diagnostic tools of the X-ray output and much more. This allows scientists using other telescopes — both on the ground and in space, including NASA’s James Webb and Hubble space telescopes — to combine this exclusive X-ray data with information from other types of light.

The richness of the Chandra Source Catalog is illustrated in a new image of the Galactic Center, the region around the supermassive black hole at the center of the Milky Way galaxy. In this image that spans just about 60 light-years across, a veritable pinprick on the entire sky, Chandra has detected over 3,300 individual sources that emit X-rays. This image is the sum of 86 observations added together, representing over three million seconds of Chandra observing time.

Another new representation of the vast scope of the Chanda Source Catalog is found in a just-released sonification, the translation of astronomical data into sound. This sonification encompasses a new map that includes 22 years of Chandra observations across the sky. Because many X-ray sources have been observed multiple times over the life of the Chandra mission, this sonification represents those repeat X-ray sightings over time through different notes.

Chandra Source Catalog Sonification. Sonification (video)
Credit: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)
[Larger Version Available Here]

In the view of the sky, projected similarly to how Earth is often depicted in world maps, the core of the Milky Way is in the center and the Galactic plane is horizontal across the middle of the image. A circle appears at the position of each detection, with the size of the circle determined by the number of detections in that location over time. A year counter appears at the top of the frame, with the text changing to “… and beyond” after 2021 as the telescope continues to collect observations. During the video, a collage of images produced by Chandra fades in as a background. In the final frames of the video, thumbnail images representing the thousands of Chandra observations taken over the lifetime of the mission appear behind the sky map.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Quick Look: NASA's Chandra Releases Deep Cut From Catalog of Cosmic Recordings

Source: NASA’s Chandra X-ray Observatory

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Visual Description:

A very deep Chandra X-ray Observatory image around the Sagittarius A* supermassive black hole, located in the center of the Milky Way galaxy, is shown. The image is dominated by burnt orange, deep gold and blue hues, with a sprinkling of rich green. The area looks both intricate and full, with a dense population of tiny dots, along with larger clumps and diffuse areas and nebulous areas peeking through.

At the center of the image, there is a bright, lumpy area in pale gold showing the intense X-ray radiation emanating from the Sagittarius A* black hole. In the surrounding area, there are more smaller lumps layered throughout, feathering out to a large almost butterfly shape filling much of the screen. The image appears textured, like dozens of blue and orange glow worms are paused in their wriggling.

The image offers an unprecedented view of lobes of hot gas extending for a dozen light years on either side of the black hole. These lobes provide evidence for powerful eruptions occurring several times over the last ten thousand years. The image also contains several mysterious X-ray filaments, some of which may be huge magnetic structures interacting with streams of energetic electrons produced by rapidly spinning neutron stars. Such features are known as pulsar wind nebulas. Chandra has detected over 3,300 individual sources that emit X-rays in this field of view. This image is the sum of 86 observations added together, representing over three million seconds of Chandra observing time.

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Fast Facts for Sagittarius A*:

Credit: NASA/CXC/SAO; Image Processing: NASA/CXC/SAO/N. Wolk
Release Date: January 23, 2026
Scale: Image is about 8.4 arcmin (63.5 light-years) across.
Category: Black Holes
Coordinates (J2000): RA 17h 45m 41.1s | Dec -29° 01´ 14.7"
Constellation: Sagittarius
Observation Dates: 86 Observations from Oct, 2000 to Jul 2020
Observation Time: 825 hours 30 minutes (2 days 7 hours 30 minutes)

Obs. ID: 1560, 1561, 2943, 2951, 2952, 2953, 2954, 3392, 3393, 3549, 3663, 3665, 4683, 4684, 5360, 5950, 5951-5954, 6113, 6363, 6639-6646, 7554-7559, 9169-9174, 10556, 11843, 13016, 13017, 14702-14704, 14941-14946, 15041-15045, 16210-16218, 16508, 16597, 16964, 16965, 18057, 18058, 18731, 18732, 19703, 19704, 20446, 20447, 20750, 20751, 22230, 22288, 23295

Instrument: ACIS/HRC
Also Known As: Galactic Center
Color Code: X-ray: red, green, and blue
Distance Estimate: About 26,000 light-years from Earth

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The Day the Sky Wouldn’t Stop Exploding: the Mystery of the Ultra-Long Gamma-Ray Burst

Artist’s impression of one possible explanation for the ultra-long gamma-ray burst GRB 250702B, showing the moment of explosion as a stellar-mass black hole merges with the massive star it is tearing apart and blasts a powerful jet into space. Image credit: NASA/LSU/Brian Monroe. Download Image

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On July 2, 2025, space telescopes monitoring the sky for brief, one-and-done flashes of high-energy light saw something that nobody expected: a gamma-ray burst (GRB) that came back again and again, stretching what is usually a single “burst” lasting seconds to minutes into an all-day event. NASA’s Fermi spacecraft triggered on multiple gamma-ray episodes from the same patch of sky over several hours, and other satellites soon reported compatible detections. Compared to the known population of GRBs that have been studied for decades, this was an outlier beast of a different species.

At first, the event’s location near the crowded plane of the Milky Way made it tempting to suspect something closer to home, located in our own Galaxy. But follow-up imaging overturned that assumption. Observations with the Very Large Telescope (VLT) in Chile narrowed down the position and, together with Hubble and JWST, revealed that the transient was coincident with a dusty, irregular host galaxy. The distance is extreme: the light from the explosion began its journey roughly 8 billion years ago. In other words, whatever happened was not a local flare—it was a truly cosmic-scale detonation, or, rather, a string of detonations.

The duration of this event was not the only weird thing about it. Archival data showed that low-energy X-rays were already present almost a day before the main gamma-ray fireworks—an “X-ray precursor” that is hard to reconcile with standard models of GRBs. Meanwhile, the gamma-ray behavior itself looked like a stuttering engine. Fermi detected a sequence of short flares separated by long gaps, collectively implying multi-hour activity from a central engine rather than the single, clean explosion typical of such events.

So, what could power an event that (1) repeats, (2) lasts for hours to a day, and (3) shows X-rays both before and after the gamma-ray fireworks? Two families of ideas have dominated the discussion. One idea keeps it in the GRB family but pushes the engine to extremes. Typical GRBs arise from the death, or collapse, of massive stars, which can produce a narrow, relativistic jet that emits gamma rays. Perhaps some aspect of the collapse, either the stellar type or the nature of the compact remnant(s) left behind could produce a central power source that simply refuses to shut off on normal timescales. The other main idea is an event completely unlike traditional GRBs and instead invokes a star wandering too close to a black hole, being torn apart, and feeding a jet aimed toward Earth. Such phenomena, known as tidal disruption events, were first predicted in the mid-1970s, but only detected twenty-five years later. Currently, we find a handful of these energetic shredding events each month, but what would make this tidal disruption event so different from the previously observed examples? The catch is that each scenario explains part of the puzzle and strains against the rest, leaving GRB 250702B as a genuine classification stress-test for high-energy astrophysics.

NuSTAR catches the engine in the act, days later

NuSTAR is built to detect high-energy X-rays, and in this event, it provided an important piece of forensic evidence. The system stayed restless well after the headline gamma-ray activity. A comprehensive X-ray campaign led by Brendan O’Connor (Instituto de Radioastronomía y Astrofísica, Mexico) combining data from the NuSTAR, Swift, and Chandra satellites found that the X-ray emission faded steeply overall. But, crucially, Swift and NuSTAR continued to detect rapid X-ray flares out to about two days after discovery. That short-timescale variability is difficult to attribute to a simple, smoothly decaying afterglow alone; instead, it points to ongoing, intermittent activity from the central engine long after standard GRB models would expect the fireworks to be over.

NuSTAR’s high-energy X-ray spectrum also helped connect competing interpretations to actual physical constraints. In the analysis jointly led by Gor Oganesyan and Annarita Ierardi (GSSI, Italy), and Elias Kammoun (Caltech, USA), the Swift lower-energy X-ray decline is shown to be extremely rapid over the first days but also shows persistent flaring activity. The NuSTAR high-energy X-ray measurement (taken about ten days after the trigger) is consistent with that same rapid fade. One idea is that this event could be associated with a "micro-tidal disruption event" in which a star was torn apart by a stellar-mass black hole, i.e., a black hole with a mass approximately ten times that of the Sun, rather than traditional tidal disruption events that involve black holes with masses thousands to millions of times that of the Sun. In short, NuSTAR did not just add data to the pot—it anchored the high-energy X-ray behavior that makes the event so hard to explain simply as a standard GRB or a standard tidal disruption event.

Where things stand now is both satisfying and unsettling. GRB 250702B is almost certainly extragalactic, almost certainly powered by a jet, and almost certainly driven by an engine that stays active far longer than a canonical GRB. But whether that engine was a star being shredded by a black hole or an unprecedented variant of a GRB progenitor remains an unanswered question, precisely because the observations pull in both directions. Resolving the origin may require what high-energy astronomers love most: the next strange event, caught early, followed deeply, and watched closely until the power source finally, and unambiguously, goes dark.

Author: Elias Kammoun (Postdoctoral Researcher, Caltech)

Full animation on NASA SVS: https://svs.gsfc.nasa.gov/14916

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/News Release

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