Mysterious object in the gap

When a black hole and a neutron star merge

On May 29, 2023, the LIGO Livingston detector observed a mysterious signal, called GW230529. It originated 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.

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, 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 on May 24, 2023, the researchers made several improvements to the detectors to increase their sensitivity.

“Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) 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 the Albert Einstein Institute 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 the Albert Einstein Institute 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 Albert Einstein Institute Potsdam were improved, so that black-hole properties can be extracted in a few days.

Fourth observing run starts with a bang

Just five days after the launch of the fourth observing run, 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 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 the Albert Einstein Institute 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 the Albert Einstein Institute 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

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

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 at the Albert Einstein Institute 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 Albert Einstein Institute Potsdam who also built the model.

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, 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 Albert Einstein Institute, 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 fourth observing run also suggests that further observations of similar signals can be expected. The 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.

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 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, the second half of O4, begins on April 10. Both LIGO detectors, Virgo, and GEO600, will participate in the second half of the run.

While the observing run continues, 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.

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

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

KAGRA is the laser interferometer with 3 km arm-length in Kamioka, Gifu, Japan. The host institute is Institute for Cosmic Ray Research, the University of Tokyo, and the project is co-hosted by National Astronomical Observatory of Japan and High Energy Accelerator Research Organization. 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 Resources for researchers are accessible from

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