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
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
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
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
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
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
The fourth observing run continues
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.
Gravitational-wave observatories
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