Astronomers have recorded the collision of a neutron star with an unknown object
On May 29, 2023, the LIGO Livingston detector observed a mysterious signal called GW230529. It resulted from the merger of a neutron star with an unknown compact object, most likely an unusually light black hole. With only a few times the mass 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 through precise waveform models, new data analysis techniques and sophisticated detector technology. Although this event was only observed using gravitational waves, it gives rise to the expectation that more similar events will be observed using electromagnetic waves in the future.
For about 30 years, researchers have debated whether there are objects whose mass falls in the “gap” between the mass of the heaviest neutron stars and the mass of the lightest black holes. Now, for the first time, scientists have discovered an object whose mass falls directly into this gap, which was thought to be practically empty. “This is a very exciting time for gravitational wave research as we dive into areas that promise to change our theoretical understanding of gravity-dominated astrophysical phenomena,” says Alessandra Buonanno, director of the Max Planck Institute for Gravitational Physics at the Potsdam Science Park.
Einstein's general theory of relativity predicts that the mass of neutron stars should not exceed three times the mass of our Sun. However, the exact maximum mass that a neutron star can have without collapsing into a black hole is unknown. “Given electromagnetic observations and our current understanding of stellar evolution, it was expected that there would be very few black holes and neutron stars in the range of three to five solar masses. However, the mass of one of the recently discovered objects falls exactly within this range,” says Buonanno.
In recent years, astronomers have discovered several objects whose mass potentially falls within this unexplored gap. In the case of GW190814, LIGO and Virgo identified an object at the lower end of the mass spectrum. However, the compact object detected by gravitational wave signal GW230529 is the first time its mass clearly falls within this gap.
New observation cycle with more sensitive detectors and improved search methods
The highly successful third observing cycle of the gravitational wave detectors concluded in the spring of 2020, bringing the number of known gravitational wave events to 90. Before the fourth observing cycle began on May 24, 2023, researchers made a number of improvements to the detectors to increase their sensitivity.
“Researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hannover, together with colleagues from LIGO, have improved the laser sources of the LIGO detectors, which are the heart of the instruments,” explains Carsten Danzmann, Director of the Albert Einstein Institute and Director of the Institute for Gravitational Physics at Leibniz University in Hanover. “They provide high-precision laser output with output power up to 125 watts with the same performance on very short and very long time scales.” Benno Wilke, head of the laser development group at the Albert Einstein Institute in Hannover, adds: “The reliability and performance of the new solid-state laser amplifiers is amazing, and I am convinced that they will also be used in the next detector upgrade.”
But it's not just the instrumentation that has been improved: the new observation cycle takes advantage of the efficient infrastructure of wave codes, and the accuracy, speed and physical content of the wave models developed at the Albert Einstein Institute in Potsdam have been improved so that the properties of black holes can be obtained in a matter of days.
Loud start of the fourth observational cycle
Just five days after the launch of the fourth observation cycle, things got really interesting: On May 29, 2023, LIGO's Livingston detector observed a gravitational wave, which was published minutes later as the candidate signal “S230529ay.” The “online analysis,” which was conducted in near real time as the signal was received, revealed that the neutron star and black hole most likely merged about 650 million light-years from Earth. However, it is impossible to say exactly where the merger occurred, since at the time of the signal, scientific data was recorded by only one gravitational wave detector. Therefore, the direction from which the gravitational waves came could not be determined.
The researchers were convinced that the signal was not a local disturbance in the LIGO Livingston detector, but actually came from deep space. “Among other things, we studied all the disturbances and random fluctuations in the detector noise, which are similar to weak signals,” explains Frank Ohme, head of the Max Planck research group at the Albert Einstein Institute in Hannover. “GW230529 clearly stands out against this background and was consistently detected by several independent search methods. This clearly indicates an astrophysical origin of the signal.”
Astrophysicists also used GW230529 to test Einstein's theory of general relativity. “GW230529 is in full agreement with the predictions of Einstein’s theory,” says Elise Zenger, a graduate student at the Albert Einstein Institute Potsdam who participated 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 encounters unknown compact object
To determine the properties of the objects that orbited each other and merged to produce the gravitational wave signal, astronomers compared data from the LIGO Livingston detector with two modern models of the waveform. “The models incorporate a number of relativistic effects to ensure that the resulting signal model is as realistic and comprehensive as possible, making it easier to compare with observational data,” says Hector Estelles Estrella, a postdoctoral fellow on the team at the Albert Einstein Institute in Potsdam who developed one of the models. “Among other things, our wave model can accurately describe black holes that rotate in spacetime at a fraction of the speed of light and emit gravitational radiation in several harmonics,” adds Lorenzo Pompili, a graduate student at the Potsdam Albert Einstein Institute, who also built this model.
GW230529 was formed as a result of the merger of a compact object whose mass is 1.3-2.1 times the mass of our Sun with another compact object whose mass is 2.6-4.7 times the Sun. 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 one is a black hole.
Thus, the mass of the heavier object lies confidently in the mass gap that was previously thought to be largely empty. No previous candidate objects in this mass range have been identified with the same confidence.
Scientists expect new observations of similar signals
Of all the neutron star-black hole mergers observed to date, GW230529 is the object in which the masses of the two objects differ the least. Tim Dietrich, professor at the University of Potsdam and leader of the Max Planck Fellows group at the Albert Einstein Institute, explains: “If a black hole is significantly heavier than a neutron star, then after the merger there is no matter left outside the black hole, and no electromagnetic radiation is emitted. Lighter black holes, “On the contrary, they can tear a neutron star apart with their more powerful tidal forces, ejecting material that can glow in the form of a kilonova or gamma-ray burst.”
The observation of such an unusual system shortly after the start of the fourth observation cycle also suggests that further observations of similar signals can be expected. The researchers calculated how often such pairs merge and found that these events occur at least as often as previously observed mergers of neutron stars with heavier black holes. Therefore, afterglow in the electromagnetic spectrum should be observed more often than previously thought.
Scientists can only speculate about how the heavier compact object—most likely a light black hole—formed in the binary that emitted GW230529. It is too light to be a direct product of a supernova. It is possible – but unlikely – that it was formed during a supernova explosion, when material initially ejected from the explosion falls back and causes the newly formed black hole to grow. It is even less likely that the black hole was formed by the merger of two neutron stars. Its origin as a primordial black hole at the dawn 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 observation cycle continues
So far, in the first half of the fourth observation cycle, 81 candidates for receiving significant signals have been identified. GW230529 is the first of these to be published after detailed study. After a few weeks of pause in commissioning and subsequent engineering launch, the second half of O4 begins on 10 April. Both LIGO detectors, Virgo and GEO600, will participate in the second half of the cycle.
While observations continue, scientists are analyzing data from O4a observations and testing the remaining 80 candidates for significant signals that have already been identified. After the break, the sensitivity of the detectors should increase slightly. It is expected that by the end of the fourth observation cycle in February 2025, the same number of new candidates will be added, and the total number of observed gravitational wave signals will soon exceed 200.
