Doubling the gravitational-wave transient catalogue

The visualization shows binary black hole mergers with parameters consistent with the 86 events from the GWTC-4.0 catalog. The tracks of the black holes are shown in white, and the gravitational-wave emission is shown in colors ranging from purple to yellow. Dark purple colors represent comparatively weak gravitational waves, whereas yellow colors represent the strongest waves emitted near the merger. The strongest gravitational waves are emitted in the directions perpendicular to the instant orbital plane. For precessing systems, the orientation of their orbital plane is constantly changing. Credit: I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer (Max Planck Institute for Gravitational Physics)

---

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

The LIGO-Virgo-KAGRA (LVK) collaboration has today released new results from the first part of the fourth observing run (O4a), which took place from May 2023 to January 2024. The scientists discovered 128 new gravitational-wave (GW) signals in the data, all of which originated from mergers of black hole and neutron star - black hole binaries. Two of the signals were observed with unprecedented clarity. Alongside releasing the strain data, the researchers have published version 4.0 of the Gravitational Wave Transient Catalogue (GWTC-4.0), which contains lists of candidate signals and measurements of their properties. The collaboration is also publishing a set of papers to accompany the catalogue. These papers have been submitted to the Astrophysical Journal Letters for publication as a Focus Issue.

The Max Planck Institute for Gravitational Physics contributed to this success

Improvements in detector sensitivity and analysis techniques led to more detections than in previous observation runs. Scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) and at Leibniz University Hannover, including many PhD students and postdoctoral researchers, have contributed to this achievement:

• Researchers at the AEI provided the high-power pre-stabilized laser system for Advanced LIGO, and have developed and tested upgrades to the main laser source currently being used in the LIGO instruments.
• The amplifier stage of the current laser sources in the Virgo and KAGRA instruments is based on developments and tests carried out by a collaboration between the AEI in Hannover and the Laser Zentrum Hannover.
• AEI researchers have developed sophisticated waveform models that are used to distinguish real cosmic sources from random fluctuations and terrestrial disturbances that appear in the detector.
• The waveform models used as templates to detect binary black holes and neutron-star—black-hole binaries were developed at the AEI. These state-of-the-art waveform models, augmented with spin-precession effects, are also employed for production runs on the signal candidates to infer their astrophysical and cosmological information.
• Another waveform model, developed at the AEI, includes the effect of mode asymmetry and the resulting 'kick', and is used in the analysis.
• Scientists at AEI have used signal candidates to search for deviations from general relativity.
• Neural network-based parameter estimation methods developed at the AEI provide a rapid and accurate way to infer the properties of binary black hole mergers.

The scientists are presenting a detailed analysis of 86 of the new signals in the catalogue, 84 of them binary black hole mergers, 2 black-hole–neutron-star mergers. In addition, the researchers found a further 42 signals that are most likely to have been produced by astrophysical sources. In total, 218 gravitational-wave candidates have been detected so far – 90 from the first three observing runs and 128 new ones.

Source: Max Planck Institute for Gravitational Physics/News

---

Movie: Visualization for GWTC-4.0

I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer (Max Planck Institute for Gravitational Physics

---

Media contacts:

Dr. Benjamin Knispel
Press Officer AEI Hannover
+49 511 762-19104
benjamin.knispel@aei.mpg.de

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

Scientific 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

Prof. Dr. Karsten Danzmann
Director | LSC Principal Investigator
Tel: +49 511 762-2356
Fax: +49 511 762-5861
karsten.danzmann@aei.mpg.de
Homepage of Karsten Danzmann

Dr. Frank Ohme
Research Group Leader | LSC Principal Investigator
Tel: +49 511 762-17171
Fax: +49 511 762-2784
frank.ohme@aei.mpg.de
Homepage of Frank Ohme

Dr. Héctor Estellés Estrella
Junior Scientist/Postdoc
Tel: +49 331 567-7193
hector.estelles@aei.mpg.de

Lorenzo Pompili
PhD Student
Tel: +49 331 567-7182
Fax: +49 331 567-7298
lorenzo.pompili@aei.mpg.de

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

Apl. Prof. Dr. Benno Willke
Group Leader
Tel: +49 511 762-2360
benno.willke@aei.mpg.de

---

Publications

1. The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration
GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog
arXiv:2508.18080 (2025)
Source | DOI

2. The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration
GWTC-4.0: Methods for Identifying and Characterizing Gravitational-wave Transients
arXiv:2508.18081 (2025)
Source |  DOI

3. The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration
GWTC-4.0: Updating the Gravitational-Wave
Transient Catalog with Observations from the First Part of the Fourth
LIGO-Virgo-KAGRA Observing Run
arXiv:2508.18082 (2025)
Source |  DOI

4. The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration
GWTC-4.0: Population Properties of Merging Compact Binaries
arXiv:2508.18083 (2025)
Source | DOI

5. The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration
Open Data from LIGO, Virgo, and KAGRA through the First Part of the Fourth Observing Run
arXiv:2508.18079 (2025)
Source | DOI

---

A new candidate for dark matter

When a superheavy, charged gravitino passes through the scintillator fluid, photons are produced that generate a characteristic “glow.” The detector should be able to detect this trace. © K. Beil, Formgeber/Milde Science Communication

---

Could traces of superheavy charged gravitinos be detected by underground detectors?

To the point:

• Big mystery: The nature of dark matter remains unclear. Possible candidates are new types of elementary particles. The present work proposes superheavy charged gravitinos to explain dark matter. These particles differ radically from all previously proposed candidates (axions, WIMPs, etc.).
• Possible detection: A research team involving the Max Planck Institute for Gravitational Physics and the University of Warsaw shows how new underground detectors could detect these particles based on their distinctive traces.
• Interdisciplinary approach: The analysis combines two very different fields of research: elementary particle physics and the search for a fundamental theory using methods of modern quantum chemistry.

In an earlier study, Hermann Nicolai from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) at Potsdam Science Park and Krzysztof Meissner from the Faculty of Physics at the University of Warsaw had already postulated superheavy electrically charged gravitinos as possible candidates for dark matter and proposed methods to search for them in planned underground experiments. The recently published paper shows how large underground neutrino detectors could detect these particles based on their distinctive traces. In the paper, the researchers present a detailed analysis of the specific signatures that events caused by gravitinos could produce at the Jiangmen Underground Neutrino Observatory (JUNO) and in future liquid argon detectors such as the Deep Underground Neutrino Experiment (DUNE). The current analysis also sets new standards in terms of interdisciplinarity by combining two very different areas of research: elementary particle physics and the search for a fundamental theory on the one hand, and methods of modern quantum chemistry on the other. The latter were contributed to this collaboration by Adrianna Kruk and Michal Lesiuk from the Faculty of Chemistry at the University of Warsaw.

An unsolved mystery

The nature of dark matter remains one of the greatest mysteries of modern astrophysics. Numerous proposals are on the table, ranging from novel elementary particles to fundamental modifications of Einstein's theory of gravity. In particle physics, supersymmetric particles, ultralight axion-like particles, and the much heavier WIMPs (weakly interacting massive particles) are discussed as possible candidates, all of which interact only very weakly with normal matter. “Many researchers had high expectations for the results of the Large Hadron Collider experiments,” says Hermann Nicolai, Director Emeritus at the AEI, “but no new particles beyond the Standard Model were detected.” Other experiments have also failed to find any evidence of such particles in this 40-year search. Nor have proposed modifications to Einstein's theory led to satisfactory answers. However, with the possible direct detection of superheavy gravitinos in underground detectors, it may now be possible to track down dark matter with a new idea.

The proposal for a dedicated search for superheavy gravitinos is based on previous work on the unification of fundamental interactions by Nicolai and his colleague Krzysztof Meissner, which could explain in particular the fermion spectrum of the Standard Model of particle physics with three generations of quarks and leptons. In this model, superheavy gravitinos (which carry spin 3/2) would be the only new fermions beyond the Standard Model. These still hypothetical elementary particles differ significantly from all previously proposed candidates. For example, a gravitino carries fractional electric charge and, in principle, can be detected directly thanks to its interaction with normal matter. However, the search is made enormously difficult by its extremely low abundance (roughly estimated to be only one gravitino per 10,000 km3 on average), which is why there is no prospect of detection with currently available detectors. However, with the commissioning of new giant underground detectors, realistic possibilities for searching for these particles are now opening up.

Superheavy gravitinos in a neutrino detector

“The observation method we propose for superheavy gravitinos is not based on ionization, as one might expect, but on a kind of ‘glow’. This glow comes from photons that should be generated when such particles pass through the detection fluid in large neutrino observatories,” says Hermann Nicolai, co-author of the study. “According to our calculations, this glow can last from a few microseconds to several hundred microseconds and would produce a characteristic trace through the detector for the superheavy gravitinos we postulate.”

Among all currently existing detectors, the Chinese JUNO underground observatory seems predestined for such a search. It aims to determine the properties of neutrinos more accurately than has been possible until now, to observe neutrinos from cosmic, atmospheric, and geological sources, and to search for new particles beyond the Standard Model. Neutrinos do not interact with electromagnetic fields and rarely react with matter. In order to observe any reactions at all, neutrino detectors must therefore have extremely large volumes. In the case of the JUNO detector, this means 20,000 tons of an organic, synthetic oil-like liquid, commonly used in chemical industry, with special additions, in a spherical vessel with a diameter of approximately 40 meters. The search for gravitinos could be conducted in parallel and independently of neutrino reactions. The quantum chemistry of the scintillator oil and its specific properties would play a central role in the predicted effect. JUNO is scheduled to begin measurements in the second half of 2025.

Unifying the forces of nature?

“The detection of the superheavy gravitinos we predicted would also be a major step forward in the search for a unified theory,” says Hermann Nicolai. “Since gravitinos are predicted to have masses on the order of the Planck mass, their detection would be the first direct indication of physics near the Planck scale and could thus provide valuable experimental evidence for a unification of the forces of nature — evidence that does not yet exist in this form.”

Source: Max Planck Institute for Gravitational Physics/News

---

Media contact:

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

Scientific contact:

Prof. Dr. Dr. h.c. Hermann Nicolai
Director emeritus
Tel:+49 331 567-7355
Fax:+49 331 567-7297
hermann.nicolai@aei.mpg.de

---

Publications

1. Kruk, A., Lesiuk, M., Meissner, K. A., Nicolai, H.
Signatures of supermassive charged gravitinos in liquid scintillator detectors
Phys. Rev. Research 7, 033145 (2025)
Source | DOI

2. Meissner, K.A., Nicolai, H.
Standard model symmetries and K(E10
J. High Energ. Phys. 2025, 54 (2025)
Source | DOI

Further information

Related press release

---

Precise modelling of high-speed black hole encounters

Visualization of the computed gravitational waves emitted in the scattering process of two black holes
Quantum Field and String Theory Group / HU

---

Applying abstract mathematical structures to real-world phenomena provides new insights into gravitational waves

An international team of researchers, including scientists from the Max Planck Institute for Gravitational Physics in the Potsdam Science Park, is setting new standards for modeling the encounter of black holes at very high speeds.

The new method is based on – so far – abstract mathematical structures, called Calabi-Yau spaces. Applying them to real astrophysical phenomena leads to highly accurate predictions of how black holes and neutron stars are deflected from their initial orbits after their encounter.

The paper, published today in Nature, comes at the right time to meet the growing demand for highly accurate theoretical predictions.

The results could be used to detect gravitational-wave signals in future observing runs of the current network of gravitational-wave detectors, with the planned third generation of ground-based observatories such as the Einstein Telescope and Cosmic Explorer, and with the space-borne Laser Interferometer Space Antenna (LISA).

Source: Max Planck Institute for Gravitational Physics/Research News

---

Media Contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
tel +49 331 567-7303
fax +49 331 567-7298
elke.mueller@aei.mpg.de

Publication

Driesse, M.; Jakobsen, G. U.; Klemm, A.; Mogull, G.; Nega, C.; Plefka, J.; Sauer, B.; Usovitsch, J.
Emergence of Calabi-Yau manifolds in high-precision black hole scattering. (2024)

---

New insights into black hole scattering and gravitational waves unveiled
Press release by Queen Mary University London

New findings on the scattering of black holes provide an important basis for understanding gravitational waves
Press release by the Humboldt University Berlin

---

Mysterious object in the gap

Inspiral of a lower mass-gap black hole (dark gray surface) and a neutron star (orange sphere). The emitted gravitational waves are shown in colors from dark blue to cyan. © I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics)

Shortly after the start of the fourth observing run, the LIGO-Virgo-KAGRA collaborations detected a remarkable gravitational-wave signal.

The LIGO Livingston detector observed the signal, called GW230529, on May 29, 2023, from the merger of a neutron star with an unknown compact object, most likely an unusually light-weight black hole. With a mass of only a few times that of our Sun, the object falls into the “lower mass gap” between the heaviest neutron stars and the lightest black holes. Researchers at the Max Planck Institute for Gravitational Physics contributed to the discovery with accurate waveform models, new data-analysis methods, and sophisticated detector technology. Although this particular event was observed only because of its gravitational waves, it increases the expectation that more such events will also be observed with electromagnetic waves in the future.

The lower mass gap

For about 30 years, researchers have debated whether there is a mass gap separating the heaviest neutron stars from the lightest black holes. Now, for the first time, LVK scientists have found an object whose mass falls right into this gap, which was thought to be almost empty. “These are very exciting times for gravitational-wave research as we delve into realms that promise to reshape our theoretical understanding of astrophysical phenomena dominated by gravity,” says Alessandra Buonanno, Director at the Max Planck Institute for Gravitational Physics in Potsdam Science Park.

Einstein's theory of general relativity predicts neutron stars to be lighter than three times the mass of our Sun. However, the exact value of the maximum mass that a neutron star can have before collapsing into a black hole is unknown. “Considering electromagnetic observations and our present grasp of stellar evolution, there were expected to be very few black holes or neutron stars within the range of three to five solar masses. However, the mass of one of the newly discovered objects precisely aligns with this range,” Buonanno elaborates.

In recent years, astronomers have uncovered several objects whose masses potentially fit within this elusive gap. In the case of GW190814, LIGO and Virgo identified an object at the lower boundary of the mass spectrum. However, the compact object detected via the gravitational-wave signal GW230529 marks the first instance where its mass unequivocally falls within this gap.

New observing run with more sensitive detectors and improved search methods

The highly successful third observing run of the gravitational-wave detectors ended in spring 2020, bringing the number of known gravitational-wave events to 90. Before the start of the fourth observing run O4 on May 24, 2023, the LVK researchers made several improvements to the detectors to increase their sensitivity. “Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Hannover, together with LIGO colleagues, have improved the laser sources of the LIGO detectors at the heart of the instruments,” explains Karsten Danzmann, Director at AEI and Director of the Institute for Gravitational Physics at Leibniz University Hannover. “They provide high-precision laser light with an output power of up to 125 watts, with the same characteristics over very short and very long time scales.” Benno Willke, leader of the laser development group at AEI Hannover, adds: “The reliability and performance of the new solid-state laser amplifiers is amazing and I'm convinced that they will still be used in the next detector upgrade.”

But not only the hardware has been improved: the new observing run took advantage of an efficient waveform code infrastructure, and the accuracy, speed, and physical content of the waveform models developed at the AEI Potsdam were improved, so that black-hole properties can be extracted in a few days.

O4 starts with a bang

Just five days after the launch of O4, things got really exciting: on May 29, 2023, the LIGO Livingston detector observed a gravitational wave that was published within minutes as signal candidate “S230529ay”. The result of this “online analysis”, which was performed almost in real time as the signal arrived, was that a neutron star and a black hole most likely merged about 650 million light-years from Earth. However, it is not possible to say exactly where the merger took place because only one gravitational-wave detector was recording scientific data at the time of the signal. Therefore, the direction from which the gravitational waves came could not be determined.

The LVK researchers made sure that the signal was not a local disturbance in the LIGO Livingston detector, but actually came from deep space. “Among other things, we examined all the perturbations and random fluctuations of detector noise that resemble weak signals,” explains Frank Ohme, leader of a Max Planck research group at AEI Hannover. “GW230529 clearly stands out from this background and was consistently detected by several independent search methods. This clearly indicates an astrophysical origin of the signal.”

The astrophysicists also used GW230529 to test Einstein's general theory of relativity. “GW230529 is in perfect agreement with the predictions of Einstein's theory,” says Elise Sänger, a graduate student at AEI Potsdam who was involved in the study. “It provided some of the best constraints to date on alternative theories of gravity using LVK gravitational-wave events.”

GW230529: Neutron star meets unknown compact object

To determine the properties of the objects that orbited each other and merged, producing the gravitational-wave signal, astronomers compared data from the LIGO Livingston detector with two state-of-the art waveform models. “The models incorporate a range of relativistic effects to ensure the resulting signal model is as realistic and comprehensive as possible, facilitating comparison with observational data,” says Héctor Estellés Estrella, a postdoctoral researcher in the team AEI Potsdam team who developed one of the models. “Among other things, our waveform model can accurately describe black holes swirling around in space-time at a fraction of the speed of light, emitting gravitational radiation across multiple harmonics,” adds Lorenzo Pompili, a PhD student at the AEI Potsdam who also built the model.

Numerical simulation of the compact binary system GW230529: Matter and waves

© I. Markin (Potsdam University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics)

GW230529 was formed by the merger of a compact object with 1.3 to 2.1 times the mass of our Sun with another compact object with 2.6 to 4.7 times the solar mass. Whether these compact objects are neutron stars or black holes cannot be determined with certainty from gravitational-wave analysis alone. However, based on all the known properties of the binary, LVK astronomers believe that the lighter object is a neutron star and the heavier is a black hole.

The mass of the heavier object therefore lies confidently in the mass gap, which was previously thought to be mostly empty. None of the previous candidates for objects in this mass range have been identified with the same certainty.

Scientists expect more observations of similar signals

Of all the neutron star-black hole mergers observed to date, GW230529 is the one in which the masses of the two objects are the least different. Tim Dietrich, a professor at the University of Potsdam and leader of a Max Planck Fellow group at the AEI, explains: “If the black hole is significantly heavier than the neutron star, no matter is left outside the black hole after the merger, and no electromagnetic radiation is emitted. Lighter black holes, on the other hand, can rip apart the neutron star with their stronger tidal forces, ejecting matter that can glow as a kilonova or a gamma-ray burst”.

The observation of such an unusual system shortly after the start of the O4 run also suggests that further observations of similar signals can be expected. The LVK researchers have calculated how often such pairs merge and found that these events occur at least as often as the previously observed mergers of neutron stars with heavier black holes. Therefore, an afterglow in the electromagnetic spectrum should be observed more frequently than previously thought.

A mysterious compact object

LVK scientists can only make an educated guess as to how the heavier of the compact objects – most likely a lightweight black hole – in the binary that emitted GW230529 was formed. It is too light to be the direct product of a supernova. It is possible – but unlikely – that it was formed during a supernova, where material initially ejected in the explosion falls back and causes the newly formed black hole to grow. It is even less likely that the black hole was formed in the merger of two neutron stars. An origin as a primordial black hole in the early days of the universe is also possible, but not very likely. Finally, the researchers cannot completely rule out the possibility that the heavier object is not a light black hole, but an extremely heavy neutron star.

The fourth observing run continues

So far, a total of 81 significant signal candidates have been identified in O4a, the first half of the fourth observing run. GW230529 is the first of these that has now been published after detailed investigation.

After a commissioning break of several weeks and a subsequent engineering run, O4b, the second half of O4, begins on April 10. Both LIGO detectors, Virgo, and GEO600, will participate in O4b.

While the observing run continues, LVK researchers are analyzing the observational data from O4a and checking the remaining 80 significant signal candidates that have already been identified. The sensitivity of the detectors should be slightly increased after the break. By the end of the fourth observing run in February 2025, a similar number of new candidates are expected to be added, and the total number of observed gravitational-wave signals will soon exceed 200.

Source: Max Planck Institute for Gravitational Physics

---

Gravitational-wave observatories

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

The Virgo Collaboration is currently composed of approximately 880 members from 152 institutions in 17 different (mainly European) countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and the National Institute for Subatomic Physics (Nikhef) in the Netherlands. A list of the Virgo Collaboration groups can be found at: 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.

---

Media contacts:

Dr. Benjamin Knispel
Press Officer AEI Hannover
+49 511 762-19104
benjamin.knispel@aei.mpg.de

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
+49 331 567-7303
+49 331 567-7298 (Fax)
elke.mueller@aei.mpg.de

---

Scientific contacts:

Prof. Dr. Alessandra Buonanno
+49 331 567-7220
+49 331 567-7298 (Fax)
alessandra.buonanno@aei.mpg.de
Homepage of Alessandra Buonanno

Prof. Dr. Karsten Danzmann
Director | LSC Principal Investigator
+49 511 762-2356
+49 511 762-5861 (Fax)
karsten.danzmann@aei.mpg.de

Homepage of Karsten Danzmann

Dr. Frank Ohme
Research Group Leader | LSC Principal Investigator
+49 511 762-17171
+49 511 762-2784 (Fax)

frank.ohme@aei.mpg.de
Homepage of Frank Ohme

Prof. Dr. Tim Dietrich
Max Planck Fellow
+49 331 567-7253
+49 331 567-7298 (Fax)
tim.dietrich@aei.mpg.de

Dr. Héctor Estellés Estrella
Junior Scientist/Postdoc
+49 331 567-7193
hector.estelles@aei.mpg.de

Lorenzo Pompili
PhD Student
+49 331 567-7182
+49 331 567-7298 (Fax)
lorenzo.pompili@aei.mpg.de

Elise Sänger
PhD Student
+49 331 567-7183
elise.saenger@aei.mpg.de

Apl. Prof. Dr. Benno Willke
Research Group Leader
+49 511 762-2360
benno.willke@aei.mpg.de

---

Publication

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration
Observation of Gravitational Waves from the Coalescence of a 2.5–4.5 M_⊙
Compact Object and a Neutron Star

Source

---

Further information

Current gravitational-wave astronomy Up-to-date information on gravitational-wave astronomy and expertise at the Max Planck Institute for Gravitational Physics in Hannover and Potsdam.

more

LIGO news item
about GW230529

---

What fuels the powerful engine of neutron star mergers?

Around sixty milliseconds after the merger, the simulation shows the jet emitted from the poles of the magnetar (up and down in this still image). The left panel shows the neutron richness of the ejected material. Blue denotes neutron-rich matter, and red denotes matter that contains neutrons and protons in roughly equal proportions. The middle panel shows surfaces of constant rest mass density. The purple curves indicate magnetic field lines. The right panel shows surfaces of constant magnetic field strength. The scale bar shows a length of 500 kilometers. Credit: Kota Hayashi (Max Planck Institute for Gravitational Physics

New computer simulation reveals the dynamo that generates large-scale magnetic fields in merging neutron stars

Merging and colliding neutron stars produce powerful kilonova explosions and gamma-ray bursts. Scientists have long suspected that a large and ultra-strong magnetic field is the engine behind these high-energy phenomena. However, the process that generates this magnetic field has been a mystery until now. Researchers at the Max Planck Institute for Gravitational Physics and at the universities in Kyoto and Toho have revealed the underlying mechanism by performing a super-high resolution computer simulation taking into account all fundamental physics. The researchers showed that ultra-strongly magnetized neutron stars, also known as magnetars, cause very bright kilonova explosions. Telescopic observations could test this prediction in the future.

Neutron stars are compact remnants of supernova explosions and consist of extremely dense matter. They are about 20 kilometers across and have up to twice the mass of our Sun, or almost 700,000 times the mass of our Earth. On August 17, 2017, astronomers observed for the first time gravitational waves, light, and gamma rays from the merger of two neutron stars. This event marked the beginning of a new kind of multi-messenger astronomy, combining gravitational-wave and electromagnetic observations.

Observations of the gravitational waves and the gamma-ray burst emitted during the merger revealed that binary neutron star mergers are the origin of at least a part of short-hard gamma-ray bursts and the heavy elements. “Only by performing a numerical simulation that takes into account all the fundamental physical effects in binary neutron star mergers will we fully understand the complete process and its underlying mechanisms,” explains Masaru Shibata, director of the Computational Relativistic Astrophysics department at the Max Planck Institute for Gravitational Physics in Potsdam. “That’s why we ran a merger simulation that took into account all the implications of Einstein’s theory of relativity and all other fundamental physics, with a spatial resolution more than ten times higher than any previous simulation, and the highest ever.”

What fuels the powerful engine of neutron star mergers?
New computer simulation reveals the dynamo that generates large-scale magnetic fields in merging neutron stars
© Kota Hayashi (Max Planck Institute for Gravitational Physics)
https://www.youtube.com/watch?v=x6qb_kt41Gs

As in the Sun so in the neutron star

High-energy phenomena associated with neutron-star mergers such as kilonova explosions and gamma-ray bursts are most likely driven by magnetohydrodynamics—the interplay between magnetic fields and fluids. This implies that a binary neutron star merger remnant must generate a strong, large-scale magnetic field via a dynamo mechanism.

“For the first time, we could pin down the physical mechanism that generates a large-scale magnetic field from smaller ones in binary neutron star mergers,” says Kenta Kiuchi, group leader in the Computational Relativistic Astrophysics department. “Part of this mechanism is the same that drives our Sun’s magnetic field. In a neutron star merger, the large-scale magnetic field emerges because of instabilities and vortices at the surface where the two neutron stars slam into one another.”

There are two phases of magnetic field amplifications: In a first phase, the Kelvin-Helmholtz instability rapidly amplifies the energy in the magnetic field by a factor of several thousand within a few milliseconds after the merger. “However, this amplified magnetic field still is a small-scale field,” explains Alexis Raboul-Salze, postdoctoral researcher in the Computational Relativistic Astrophysics department. “But after a few milliseconds, there is a second phase of magnetic field amplification due to another instability, the magnetorotational instability. This instability further amplifies the small-scale field and acts as a dynamo on the large-scale field – the same mechanism as in the Sun.”

The resulting highly magnetized massive neutron star born in the collision is hypothetically proposed as a magnetar. About 40 milliseconds after the merger the magnetic fields drives a strong particle wind at relativistic speeds from the poles of the magnetar. This wind forms a jet, which is related to the observed high-energy phenomena. The research group shows that this hypothesis is feasible for the first time.

“Our simulation suggests that the magnetar engine generates very bright kilonova explosions. We can test our prediction by multi-messenger observations in the near future,” concludes Masaru Shibata.

Source: Max Planck Institute for Gravitational Physics

---

Media contact:

Dr. Elke Müller
Press Officer AEI
Potsdam, Scientific Coordinator
tel:+49 331 567-7303
tel:+49 331 567-7298
elke.mueller@aei.mpg.de

Scientific contacts:

Prof. Masaru Shibata
Director
tel:+49 331 567-7222
tel:+49 331 567-7298
masaru.shibata@aei.mpg.de

Dr. Kenta Kiuchi
Group Leader
tel:+49 331 567-7320
kenta.kiuchi@aei.mpg.de

Dr. Alexis Reboul-Salze
Junior Scientist/Postdoc
tel:+49 331 567-7234
alexis.reboul-salze@aei.mpg.de

---

Publication:

Kenta Kiuchi, Alexis Reboul-Salze, Masaru Shibata, Yuichiro Sekiguchi
A large-scale magnetic field produced by a solar-like dynamo in binary neutron star mergers
Nature Astronomy (2024)

Source | DOI

---

Lightest black hole or heaviest neutron star?

An artist’s impression of the system assuming that the massive companion star is a black hole. The brightest background star is its orbital companion, the radio pulsar PSR J0514-4002E. The two stars are separated by 8 million kilometers and circle each other every 7 days. MPIfR; Daniëlle Futselaar (artsource.nl)

MeerKAT uncovers a mysterious object at the boundary between black holes and neutron stars

An international team of astronomers, led by researchers from the Max Planck Institute for Radio Astronomy, have used the MeerKAT radio telescope to discover an intriguing object of an unknown nature in the globular cluster NGC 1851. The massive object is heavier than the heaviest neutron stars known and yet simultaneously lighter than the lightest black holes known and is in orbit around a rapidly spinning millisecond pulsar. This could be the first discovery of the much-coveted radio pulsar - black hole binary; a stellar pairing that would allow new tests of Einstein’s general relativity. The research was published today in the journal Science.

Neutron stars, the ultra-dense remains of a supernova explosion, can only be so heavy. Once they’ve acquired too much mass, perhaps by consuming another star or maybe by colliding with another of their kind, they will collapse. What exactly they become once they collapse is the cause of much speculation, with various wild and wonderful flavours of exotic stars being proposed. The prevailing opinion, however, is that neutron stars collapse to become black holes, objects so gravitationally attractive that even light cannot escape them. Theory, backed by observation, tells us that the lightest black holes that can be created by collapsing stars are about 5 times more massive than the Sun. This is considerably larger than the 2.2 times the mass of the sun required for neutron star collapse, giving rise to what is known as the black hole mass gap. The nature of compact objects in this mass gap is unknown and detailed study has thus far proved challenging due to only fleeting glimpses of such objects being caught in observations of gravitational-wave merger events in the distant universe.

The team used the sensitive MeerKAT radio telescope, located in the Karoo semi-desert in South Africa
Credit: SARAO

Discovery in the mass gap

The discovery of an object in this mass-gap in our own galaxy by a team of astronomers from the international Transients and Pulsars with MeerKAT (TRAPUM) collaboration may help finally understand these objects. Their work, published this week in the journal Science, reports on a massive pair of compact stars in the globular cluster NGC 1851 in the southern constellation Columba (the dove). By using the sensitive MeerKAT radio telescope in South Africa, in combination with powerful instrumentation built by engineers at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, they were able to detect faint pulses from one of the stars, identifying it as a radio pulsar, a type of neutron star that spins rapidly and shines beams of radio light into the Universe like a cosmic lighthouse.

This pulsar, designated PSR J0514-4002E, spins more than 170 times a second, with every rotation producing a rhythmic pulse, like the ticking of a clock. By observing small changes in this ticking over time, using a technique called pulsar timing, they were able to make extremely precise measurements of its orbital motion. “Think of it like being able to drop an almost perfect stopwatch into orbit around a star almost 40,000 light years away and then being able to time those orbits with microsecond precision,” says Ewan Barr, who led the study together with MPIfR colleague and PhD candidate Arunima Dutta.

An invisible partner

“By regularly timing the pulsar and carefully analyzing our observations, we were able to precisely pinpoint the pulsar’s location. But when we looked at Hubble images of NGC 1851, we saw nothing at that position,” explains Prajwal Voraganti Padmanabh, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hannover. “Hence, the object in orbit with the pulsar is not a normal star, but an extremely dense remnant of a collapsed star.” Furthermore, the observed change with time of the closest point of approach between the two stars (the periastron) showed that the companion has a mass that was simultaneously bigger than that of any known neutron star and yet smaller than that of any known black hole, placing it squarely in the black-hole mass gap.

“Whatever this object is, it is exciting news”, says Paulo Freire, of the MPIfR. “If it is a black hole, it will be the first pulsar - black hole system known, which has been a Holy Grail of pulsar astronomy for decades! If it is a neutron star, this will have fundamental implications for our understanding of the unknown state of matter at these incredible densities.”

A possible evolutionary history for the NGC 1851E system is captured in this figure. On the left of the figure it is shown how the millisecond pulsar (MSP), PSR J0514-4002E, was spun up via the capture of matter from a stellar companion in a low-mass x-ray binary (LMXB). What remains after the low-mass x-ray binary stage, is a rapidly spinning pulsar and a white dwarf that orbit each other - a typical configuration seen throughout the Galaxy. On the right the formation of the massive companion object is shown. Here two neutron stars in orbit (NS + NS) can be seen. The loss of energy by gravitational wave emission causes this orbit to shrink with time, eventually resulting in an explosive neutron star merger. The outcome of the merger is an isolated low-mass black hole (BH) or possibly a supermassive neutron star. At some later time the black hole and the pulsar - white dwarf binary meet, resulting in an exchange encounter in which the lightest of the three stars (in this case the white dwarf) is kicked out of orbit. The result is a stable pulsar - black hole system. Credit: Thomas Tauris (Aalborg University / MPIfR)

The most exotic binary pulsar discovered yet

The team proposes that the formation of the massive object, and its subsequent pairing with the fast-spinning radio pulsar in a tight orbit, is the result of a rather exotic formation history only possible due to its particular local environment. The system is found in the globular cluster NGC 1851, a dense collection of old stars that are much more tightly packed than the stars in the rest of the Galaxy. Here, it is so crowded that the stars can interact with each other, disrupting orbits and in the most extreme cases colliding. It is one such collision between two neutron stars that is proposed to have created the massive object that now orbits the radio pulsar. However, before the present binary was created, the radio pulsar must have first acquired material from a donor star in a so-called low-mass X-ray binary. Such a “recycling” process is needed to spin up the pulsar to its current rotation rate. The team believes that this donor star was then replaced by the current massive object in a so-called exchange encounter. “This is the most exotic binary pulsar discovered yet,” says Thomas Tauris from Aalborg University, Denmark. “Its long and complex formation history pushes at the limits of our imagination.”

While the team cannot conclusively say whether they have discovered the most massive neutron star known, the lightest black hole known or even some new exotic star variant, what is certain is that they have uncovered a unique laboratory for probing the properties of matter under the most extreme conditions in the Universe.

“We're not done with this system yet,“ says Arunima Dutta. She concludes, “uncovering the true nature of the companion will a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap”.

Source: Max Planck Institute for Gravitational Physics/News

---

Media contact:

Dr. Benjamin Knispel
Press Officer AEI Hannover
+49 511 762-19104
benjamin.knispel@aei.mpg.de

Dr. Prajwal Voraganti Padmanabh
Junior Scientist/Postdoc
+49 511 762-17024
prajwal.voraganti.padmanabh@aei.mpg.de

---

Publication

E. Barr et al.
A pulsar in a binary with a compact object in the mass gap between neutron stars and black holes
Science Vol 383, Issue 6680, pp. 275-279 (2024)
Source | DOI

---

Further information

Homepage of the “Pulsars” research group

This group's main research aspects are computing-intense searches for and studies of pulsars – rapidly spinning neutron stars – through gamma rays and radio waves in previously inaccessible parameter spaces using efficient data analysis and powerful computing resources.

More

---

Fundamental Physics in Radio Astronomy at MPIfR

TRAPUM homepage
Information about the Transients and Pulsars with MeerKAT Project

---

Studying neutron stars on many channels in parallel

Numerical simulation of the resulting ejecta material of two merging neutron stars. Red colors refer to ejected material with a high fraction of neutrons which will appear typically redder than blue material that contains a higher fraction of protons. © I. Markin (University of Potsdam)

International research team succeeds for the first time in analyzing very different signals simultaneously

An international team of researchers, including the Max Planck Institute for Gravitational Physics and the University of Potsdam, has developed a method to analyze most of the observable signals associated with neutron star mergers simultaneously. For the first time, it was possible to model and interpret the emitted gravitational waves, the kilonova, and the afterglow of the gamma-ray burst of the merger of two neutron stars observed on August 17, 2017. The study and the code infrastructure developed for it provide precise information about the properties of nuclear matter and form the basis for the analysis of future events. The research results have now been published in the journal Nature Communications.

“Our new method will help to analyze the properties of matter at extreme densities. It will also allow us to better understand the expansion of the universe and to what extent heavy elements are formed during neutron star mergers,” explains Tim Dietrich, Professor at the University of Potsdam and leader of a Max Planck Fellow group at the Max Planck Institute for Gravitational Physics. Dietrich is corresponding author of the paper.

Extreme conditions in a cosmic laboratory

A neutron star is a superdense astrophysical object formed at the end of a massive star's life in a supernova explosion. Like other compact objects, some neutron stars orbit each other in binary systems. They lose energy through the constant emission of gravitational waves – tiny ripples in the fabric of space-time – and eventually collide. Such mergers allow researchers to study physical principles under the most extreme conditions in the universe. For example, the conditions of these high-energy collisions lead to the formation of heavy elements such as gold. Indeed, merging neutron stars are unique objects for studying the properties of matter at densities far beyond those found in atomic nuclei.

The new method was applied to the first and so far only multi-messenger observation of binary neutron star mergers. In this event, discovered on August 17, 2017, the stars' last few thousand orbits around each other had warped space-time enough to create gravitational waves, which were detected by the terrestrial gravitational-wave observatories Advanced LIGO and Advanced Virgo. As the two stars merged, newly formed heavy elements were ejected. Some of these elements decayed radioactively, causing the temperature to rise. Triggered by this thermal radiation, an electromagnetic signal in the optical, infrared, and ultraviolet was detected up to two weeks after the collision. A gamma-ray burst, also caused by the neutron star merger, ejected additional material. The reaction of the neutron star's matter with the surrounding medium produced X-rays and radio emissions that could be monitored on time scales ranging from days to years.

More accurate results for future detections

The new tool for simultaneously analyzing astrophysical data from different sources allows researchers to interpret all these signals at the same time and to incorporate additional information from radio and X-ray observations of neutron stars (e.g., from NASA's NICER telescope), from nuclear physics calculations, and even from heavy-ion collision experiments at accelerators on Earth. "We can now go beyond the usual step-by-step combination process that we have done before. By analyzing coherently and simultaneously, we get more precise results," says Peter T. H. Pang, scientist at Utrecht University, first author of the paper and lead developer of the code. To even further improve the developed software over the coming years, Dietrich was awarded with an ERC Starting Grant worth 1.5 million euros in 2022.

The gravitational-wave detectors are currently in their fourth observing run. The next detection of a neutron star merger could come any day, and the researchers are eagerly waiting to use the tool they developed again.

Source: Max Planck Institute for Gravitational Physics/News

---

Media contact:

Dr. Elke Müller
Press Officer AEI
Potsdam, Scientific Coordinator
tel:+49 331 567-7303
tel:+49 331 567-7298
elke.mueller@aei.mpg.de

Science contact:

Prof. Dr. Tim Dietrich
Max Planck Fellow
tel:+49 331 567-7253
tel:+49 331 567-7298
tim.dietrich@aei.mpg.de

---

Publication

Peter T. H. Pang, Tim Dietrich, Michael W. Coughlin, Mattia Bulla, Ingo Tews, Mouza Almualla, Tyler Barna, Ramodgwendé Weizmann Kiendrebeogo, Nina Kunert, Gargi Mansingh, Brandon Reed, Niharika Sravan, Andrew Toivonen, Sarah Antier, Robert O. VandenBerg, Jack Heinzel, Vsevolod Nedora, Pouyan Salehi, Ritwik Sharma, Rahul Somasundaram, Chris Van Den Broeck

An updated nuclear-physics and multi-messenger astrophysics framework for binary neutron star mergers
Nature Communications, Vol. 14, p. 1-13 (2023)

Source

---

For whom the black hole rings

The three phases of a black hole collision. After an initial inspiral, the merger follows. The newly formed still asymmetric black hole then emits rapidly fading gravitational waves. Adapted from: Observation of Gravitational Waves from a Binary Black Hole Merger, B. P. Abbott et al. (LIGO ScientificCollaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 061102, https://doi.org/10.1103/PhysRevLett.116.061102

Observation of multiple ringdown modes in a black hole merger

An international team led by researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover has found strong observational evidence for multiple gravitational-wave frequencies in a binary black hole ringdown. The team discovered that the intermediate-mass black hole formed in the GW190521 event vibrated briefly at at least two frequencies after the merger. This ringdown is a fundamental prediction from general relativity. Its observation allows tests of the theory and of the black hole no-hair theorem. The scientists found no violations of the theorem or deviations from general relativity. It was widely assumed that this observation of multiple tones would be impossible before the next generation of gravitational-wave detectors. Nevertheless, the unexpected massive merger remnant of GW190521 together with exquisite data analysis methods make the detection possible. The results were published in Physical Review Letters.

When two black holes collide, gravitational waves are emitted in three phases: when they inspiral, when they merge, and when the newly formed initially lopsided black hole settles into its final stage. The last phase, called “ringdown”, is a fraction-of-a-second period of black hole vibrations that – according to Einstein’s theory of general relativity – encode information about the mass and the spin of the final black hole.

A black hole rings like a bell

“The black hole is similar to a bell that rings, producing a spectrum of multiple fading tones, that encode information about the bell,” explains Collin Capano, corresponding author of the study published in Physical Review Letters and formerly a researcher in the Observational Relativity and Cosmology department at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover.

An international team led by researchers from the AEI Hannover analyzed public LIGO and Virgo data from the GW190521 event, which is one of the most massive binary black hole mergers observed to date. They discovered a chord of two damped tones (also called “quasi-normal modes”) present in the gravitational waves emitted during the ringdown phase of the event.

This diagram shows the frequencies (horizontal axis) and damping times (vertical axis) of the two tones observed in the decay of GW190521 as colored areas. The search found two tones consistent with the predictions of general relativity.

No hair on GW190521

Numerical simulation of a heavy black-hole merger (GW190521)

 © N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes (SXS) Collaboration

The no-hair theorem states that in general relativity black holes are completely characterized by three externally accessible quantities: their mass, spin, and electric charge, although charge is negligible for astrophysical black holes. No further information, or additional “hair” is required to describe them. The frequencies of the ringdown modes and their damping times of the black hole formed in GW190521 must therefore be determined by mass and spin only.

“We tested the black hole no-hair theorem by comparing the frequencies and the damping times of the two modes we found in the GW190521 ringdown,” says Julian Westerweck, co-author of the publication and a former PhD student in the Observational Relativity and Cosmology department at AEI Hannover. “GW190521 passed the test and we found no signs of any black hole physics beyond Einstein’s general theory of relativity. It is quite remarkable that a theory that is over one hundred years old now continues to work so well.”

The research team assumed that the frequency and decay time of the fundamental vibration mode of the black hole depend on its mass and spin as predicted by Einstein’s theory. They allowed the frequency and decay time of the second mode to deviate from the values expected in general relativity and checked how well such deviations fit the observations. Their analysis found no such deviations and showed that GW190521 is consistent with Einstein’s theory.

The results also exclude two alternative proposals about the somewhat mysterious nature of GW190521. Both a head-on collision of exotic stars and the collapse of a massive star to a black hole with a high-mass disk are not compatible with the observed multimodal ringdown.

“More than 20 years ago, we had proposed such observations as a means of testing the nature of black holes” says Badri Krishnan, co-author, long-term visitor and former staff member at AEI Hannover, currently professor at Radboud University. “At the time we did not believe that the LIGO and Virgo detectors would be able to observe multiple ringdown modes. Therefore these results are particularly gratifying for me.”

Source: Max Planck Institute for Gravitational Physics

---

Media contact:

Dr. Benjamin Knispel
Press Officer AEI Hannover
tel:+49 511 762-19104
benjamin.knispel@aei.mpg.de

Science contacts:

Dr. Collin Capano

High Performance Computing Facilitator
ccapano@umassd.edu
Center for Computing and Data Science Research at the University of Massachusetts, Dartmouth

Dr. Julian Westerweck
Research Fellow in Gravitational Wave Physics
j.m.westerweck@bham.ac.uk
University of Birmingham

Dr. Badri Krishnan
Long-term visitor
tel:+49 511 762-17134
badri.krishnan@aei.mpg.de

Publication:

Capano, C.; Cabero, M.; Abedi, J.; Kastha, S.; Westerweck, J.; Nitz, A. H.; Nielsen, A. B.; Krishnan, B.
Multimode Quasinormal Spectrum from a Perturbed Black Hole
Phys. Rev. Lett. 131, 221402 (2023)
Source | DOI

Supplementary data
for the publication “Observation of a multimode quasi-normal spectrum from a perturbed black hole”

---

Understanding neutron star mergers

Numerical-relativistic simulation of a binary neutron star merger

The neutron stars have masses of 1.2 and 1.5 solar masses, respectively, which is consistent with the parameters of the merger observed in August 2017 (GW170817). The data was generated during a one second-long general-relativistic neutrino-radiation magnetohydrodynamic simulation. The visualization shows the electron fraction on the left, the density in the center, and the magnetic field strength (1015 Gauss) on the right. ©K. Hayashi, K. Kiuchi (Max Planck Institute for Gravitational Physics & Kyoto University). Video YouTube

One second-long numerical-relativistic simulation of a binary neutron star merger

One second-long numerical-relativistic simulation of a binary neutron star merger The neutron stars have masses of 1.2 and 1.5 solar masses, respectively, which is consistent with the parameters of the merger observed in August 2017 (GW170817). The visualization shows profiles for rest-mass density (top-left), magnetic-field strength (top-second from left), magnetization parameter (top-second from right), unboundedness defined by the Bernoulli criterion (top-right), electron fraction (bottom-left), temperature (bottom-second from left), entropy per baryon (bottom-second from right), and Shakura-Sunyaev αM parameter (bottom-right). ©K. Hayashi, K. Kiuchi (Max Planck Institute for Gravitational Physics & Kyoto University).. Video YouTube

---

Complex numerical simulation sheds light on an extreme cosmic process

Scientists from the Max Planck Institute for Gravitational Physics in Potsdam and from the Universities of Kyoto and Toho have succeeded for the first time in studying the entire process of two neutron stars orbiting and merging with each other in a long numerical-relativistic simulation. Until now, only simulations describing about 0.1 seconds of the entire process were feasible for such binary systems. The new modeling, which took the Japanese high-performance computer Fugaku 200 days to compute, maps a time span ten times longer and provides new insights into the formation of heavy elements. The study has now appeared in the journal Physical Review Letters.

When in August 2017 gravitational waves from two merging neutron stars were detected, it was a scientific sensation and the beginning of multimessenger astronomy, which combines measurements from gravitational-wave detectors with observations from telescopes that pick up signals in the electromagnetic range. However, it is still not known what exactly happens during and after such a merger. To learn more about neutron stars, which typically have 40% more mass than our Sun at a diameter of about 20 kilometers, highly accurate theoretical computations are needed. The two neutron stars in the simulation published today had 1.2 and 1.5 solar masses, respectively, which is consistent with the parameters of the merger observed in August 2017.

Scientists expect to observe coalescing neutron stars also during the recently started fourth observation run of the gravitational-wave detectors. For the interpretation of such signals, reliable theoretical predictions are crucial, which are now available for the first time. "Until now, it has always been necessary to combine different methods to model the complete process of the inspiral, merger and the post-merger phase," explains Kenta Kiuchi, group leader in the Computational Relativistic Astrophysics Department at the Max Planck Institute for Gravitational Physics and first author of the paper. "In addition, previous studies included a lot of assumptions that were not always physically motivated. Our study, on the other hand, is self-consistent and making only a few assumptions. It is the first complete computation of the entire process and provides an accurate picture of the mass ejection during and shortly after the binary neutron star merger."

The formation of heavy elements

The study took 72 million CPU hours on the Japanese high-performance computer Fugaku to simulate one crucial second of the entire process: the last five orbits, the merger itself, and the phase afterwards. "With this long simulation, we have learned a lot about the physics of neutron star coalescences," says Masaru Shibata, director of the Computational Relativistic Astrophysics Department. "It's becoming more and more clear that the elements heavier than iron are in fact synthesized in such extremely energetic processes when matter is ejected from the system during and after the merger."

The researchers closely studied the ejection of mass from the system and found that matter is ejected starting about 10 milliseconds after the merger. After 40 milliseconds, this dynamic mass ejection peaks, then flattens out, and about 300 milliseconds after the merger, matter is again ejected – this time from the torus that formed during the merger. While the dynamic mass ejection is due to tidal forces and shock heating during the merger, ejection of matter after the merger results from turbulence in the torus as the scientists have now been able to self-consistently show for the first time.

 

Source:  Max Planck Institute for Gravitational Physics

---

Media contact:

Dr. Elke Müller
Press Officer AEI
Potsdam, Scientific Coordinator
tel.+49 331 567-7303
tel.+49 331 567-7298
elke.mueller@aei.mpg.de

Science contacts:

Dr. Kenta Kiuchi
Group Leader
tel.+49 331 567-7320
kenta.kiuchi@aei.mpg.de

Prof. Masaru Shibata
Director
tel.+49 331 567-7222
tel.+49 331 567-7298
masaru.shibata@aei.mpg.de

Publication:

Kenta Kiuchi, Sho Fujibayashi, Kota Hayashi, Koutarou Kyutoku, Yuichiro Sekiguchi, Masaru Shibata

Self-consistent picture of the mass ejection from a one second-long binary neutron star merger leaving a short-lived remnant in general-relativistic neutrino-radiation magnetohydrodynamic simulation

Phys. Rev. Lett. 131, 011401 (2023)

Source | DOI

---

Towards the detection of the nanohertz gravitational-wave background

Fig. 1: Artistic impression of the EPTA experiment. A coordinated network of European radio telescopes observed an array of pulsars distributed across the sky. The measured variation in the arrival time of the emitted pulses on Earth allows astronomers to study tiny variations in spacetime. These variations, called gravitational waves, still propagate the Universe from a distant past, when galaxies merged and the supermassive black holes in their centre orbited each other with a period of a few years to produce them. © M. Kramer (Max Planck Institute for Radio Astronomy)

Fig. 2: The Five Major European Radio Telescopes from left top to right bottom: Effelsberg Radio Telescope, Germany, Nancay Radio Telescope, France, Sardinia Radio Telescope, Italy, Westerbork Synthesis Radio Telescope, The Netherlands and Lovell Telescope, UK. © N. Tacken/Max Planck Institute for Radio Astronomy (Effelsberg), Letourneur and Nançay Observatory (Nançay), A. Holloway (Jodrell Bank), ASTRON (WSRT), G. Alvito/INAF (SRT)

The European Pulsar Timing Array provides a significant step forward

The European Pulsar Timing Array (EPTA) is a scientific collaboration bringing together teams of astronomers around the largest European radio telescopes, as well as groups specialized in data analysis and modelling of gravitational-wave (GW) signals. It has published a detailed analysis of a candidate signal for the since-long sought gravitational-wave background (GWB) due to in-spiraling supermassive black-hole binaries. Although a detection cannot be claimed yet, this represents another significant step in the effort to finally unveil GWs at very low frequencies, of order one billionth of a Hertz. In fact, the candidate signal has emerged from an unprecedented detailed analysis and using two independent methodologies. Moreover, the signal shares strong similarities with those found from the analyses of other teams.

The results were made possible thanks to the data collected over 24 years with five large-aperture radio telescopes in Europe (see Fig. 2). They include MPIfR’s 100-m Radio Telescope near Effelsberg in Germany, the 76-m Lovell Telescope in Cheshire/United Kingdom, the 94-m Nançay Decimetric Radio Telescope in France, the 64-m Sardinia Radio Telescope at Pranu Sanguni, Italy and the 16 antennas of the Westerbork Synthesis Radio Telescope in the Netherlands. In the observing mode of the Large European Array for Pulsars (LEAP), the EPTA telescopes are tied together to synthesize a fully steerable 200-m dish to greatly enhance the sensitivity of the EPTA towards gravitational waves.

Radiation beams from the pulsars’ magnetic poles circle around their rotational axes, and we observe them as pulses when they pass our line of sight, like the light of a distant lighthouse. Pulsar timing arrays (PTAs) are networks of very stably rotating pulsars, used as galactic-scale GW detectors. In particular, they are sensitive to very low frequency GWs in the billionth-of-a-Hertz regime. This will extend the GW observing window from the high frequencies (hundreds of Hertz) currently observed by the ground-based detectors LIGO/Virgo/KAGRA. While those detectors probe short lasting collisions of stellar-mass black holes and neutron stars, PTAs can probe GWs such as those emitted by systems of slowly in-spiraling supermassive black-hole binaries hosted at the centres of galaxies. The addition of the GWs released from a cosmic population of these binaries forms a GWB.

Dr. Jonathan Gair, Group Leader in the “Astrophysical and Cosmological Relativity” department at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam and co-author of the study says: “In analysing pulsar timing data, we are looking for a common red noise in the pulsars that is caused by a gravitational wave background. The fact that we are seeing such a red noise is very exciting, but we cannot yet say that it is caused by gravitational waves. The astrophysical implications of the detection of a gravitational wave background from a population of supermassive black holes would be profound. The amplitude and properties of this background are affected by the process through which galaxies assemble and massive black hole binaries form and merge.”

However, the amplitude of the red noise is incredibly tiny (estimated to be tens to a couple hundreds of a billionth of a second) and in principle many other effects could impart that to any given pulsar in the PTA.

To validate the results, multiple independent codes with different statistical frameworks were then used to mitigate alternate sources of noise and search for the GWB.
Importantly, two independent end-to-end procedures were used in the analysis for cross-consistency. Additionally, three independent methods were used to account for possible systematics in the Solar-system planetary parameters used in the models predicting the pulse arrival times, a prime candidate for false-positive GW signals.

The EPTA analysis with both procedures found a clear candidate signal for a GWB and its spectral properties (i.e. how the amplitude of the observed noise varies with its frequency) remain within theoretical expectations for the noise attributable to a GWB.

Dr. Nicolas Caballero, researcher at the Kavli Institute for Astronomy and Astrophysics in Beijing and co-lead author explains: “The EPTA first found indications for this signal in their previously published data set in 2015, but as the results had larger statistical uncertainties, they were only strictly discussed as upper limits. Our new data now clearly confirm the presence of this signal, making it a candidate for a GWB“.

Einstein’s General Relativity predicts a very specific relation among the spacetime deformations experienced by the radio signals coming from pulsars located in different directions in the sky. Scientists call that as the spatial correlation of the signal, or Hellings and Downs curve. Its detection will uniquely identify the observed noise as due to a GWB. Dr. Siyuan Chen, researcher at the LPC2E, CNRS in Orleans, co-lead author of the study, notes “At the moment, the statistical uncertainties in our measurements do not allow us yet to identify the presence of spatial correlation expected for the gravitational-wave background signal. For further confirmation we need to include more pulsar data into the analysis, however the current results are very encouraging“.

The EPTA is a founding member of the International Pulsar Timing Array (IPTA). As analyses of independent data performed by the other IPTA partners (i.e. the NANOGrav and the PPTA experiments) also indicated similar common signals, it has become vital to apply multiple analysis algorithms to increase confidence in a possible future GWB detection. The IPTA members are working together, drawing conclusions from comparing their data and analyses to better prepare for the next steps.

“As the signal we are looking for is stochastic, it is easy to confuse it with other random processes occurring in the pulsars or in the instruments used to observe them”, says Lorenzo Speri, PhD student in the “Astrophysical and Cosmological Relativity” department and co-author of the study. “Separating the common red noise, which is our signal, from individual noises, requires careful statistical analysis.” And Jonathan Gair adds: “The statistical techniques have been carefully developed over the last decade and it is gratifying to see them finally yielding promising scientific results.”

Jonathan Gair has been a member of the EPTA for the past decade, working on developing the statistical formalism used to analyse EPTA data. Lorenzo Speri has been working on the analysis of EPTA data for the past year, including, among other things, the optimal selection of pulsars to use in the analysis. Over the coming year, Gair and Speri will be working on the analysis of the full EPTA data set, and the IPTA data set that combines this data with data from other pulsar timing collaborations. The hope is that these data sets, containing many more pulsars, might have sufficient sensitivity that the origin of the background may begin to be identified.

---

Media contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
tel:+49 331 567-7303
tel:+49 331 567-7298

Science contacts:

Dr. Jonathan Gair
Group Leader
tel:+49 331 567-7306
tel:+49 331 567-7298

Lorenzo Speri
PhD Student
tel:+49 331 567-7185
tel:+49 331 567-7298

---

Publication:

1. Chen, S.; Caballero, R. N.; Guo, Y. J.; Chalumeau, A.; Liu, K.; Shaifullah, G.; Lee, K. J.; Babak, S.; Desvignes, G.; Parthasarathy, A.; Hu, H.; van der Wateren, E.; Antoniadis, J.; Bak Nielsen, A.-S.; Bassa, C. G.; Berthereau, A.; Burgay, M.; Champion, D. J.; Cognard, I.; Falxa, M.; Ferdman, R. D.; Freire, P. C. C.; Gair, J. R.; Graikou, E.; Guillemot, L.; Jang, J.; Janssen, G. H.; Karuppusamy, R.; Keith, M. J.; Kramer, M.; Liu, X. J.; Lyne, A. G.; Main, R. A.; McKee, J.W.; Mickaliger, M. B.; Perera, B. B. P.; Perrodin, D.; Petiteau, A.; Porayko, N. K.; Possenti, A.; Samajdar, A.; Sanidas, S. A.; Sesana, A.; Speri, L.; Stappers, B. W.; Theureau, G.; Tiburzi, C.; Vecchio, A.; Verbiest, J. P. W.; Wang, J.; Wang, L. and Xu, H.

Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array: Inferences in the stochastic gravitational-wave background search Monthly Notices of the Royal Astronomical Society 508, 4970 (2021)

Source / DOI

---

Further Information:

Science Summary of the publication

European Pulsar Timing Array

Participating telescopes

• Effelsberg Radio Telescope
• Lovell Telescope
• Nançay Decimetric Radio Telescope
• Sardinia Radio Telescope
• Westerbork Synthesis Radio Telescope

Source: Max Planck Institute for Gravitational Physics/News

---