Observation of Gravitational Waves from the Coalescence of a 2.5-4.5 M⊙ Compact Object and a Neutron Star
The LIGO-Virgo-KAGRA collaboration reported the detection of gravitational waves from the merger of a neutron star and a compact object with a mass between 2.5 and 4.5 solar masses, providing evidence for the existence of objects in the lower mass gap and suggesting that such neutron star-black hole mergers constitute a significant portion of the total merger rate.
Original authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, R. Abbott, I. Abouelfettouh, F. Acernese, K. Ackley, S. Adhicary, N. Adhikari, R. X. Adhikari, V. K. Adkins, D. Agarwal, M. Agathos, M. Aghaei Abchouyeh, O. D. Aguiar, I. Aguilar, L. Aiello, A. Ain, P. Ajith, S. Akçay, T. Akutsu, S. Albanesi, R. A. Alfaidi, A. Al-Jodah, C. Alléné, A. Allocca, S. Al-Shammari, P. A. Altin, S. Alvarez-Lopez, A. Amato, L. Amez-Droz, A. Amorosi, C. Amra, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Andia, M. Ando, T. Andrade, N. Andres, M. Andrés-Carcasona, T. Andrić, J. Anglin, S. Ansoldi, J. M. Antelis, S. Antier, M. Aoumi, E. Z. Appavuravther, S. Appert, S. K. Apple, K. Arai, A. Araya, M. C. Araya, J. S. Areeda, L. Argianas, N. Aritomi, F. Armato, N. Arnaud, M. Arogeti, S. M. Aronson, K. G. Arun, G. Ashton, Y. Aso, M. Assiduo, S. Assis de Souza Melo, S. M. Aston, P. Astone, F. Attadio, F. Aubin, K. AultONeal, G. Avallone, D. Azrad, S. Babak, F. Badaracco, C. Badger, S. Bae, S. Bagnasco, E. Bagui, J. G. Baier, L. Baiotti, R. Bajpai, T. Baka, M. Ball, G. Ballardin, S. W. Ballmer, S. Banagiri, B. Banerjee, D. Bankar, P. Baral, J. C. Barayoga, B. C. Barish, D. Barker, P. Barneo, F. Barone, B. Barr, L. Barsotti, M. Barsuglia, D. Barta, A. M. Bartoletti, M. A. Barton, I. Bartos, S. Basak, A. Basalaev, R. Bassiri, A. Basti, D. E. Bates, M. Bawaj, P. Baxi, J. C. Bayley, A. C. Baylor, P. A. Baynard, M. Bazzan, V. M. Bedakihale, F. Beirnaert, M. Bejger, D. Belardinelli, A. S. Bell, V. Benedetto, W. Benoit, I. Bentara, J. D. Bentley, M. Ben Yaala, S. Bera, M. Berbel, F. Bergamin, B. K. Berger, S. Bernuzzi, M. Beroiz, C. P. L. Berry, D. Bersanetti, A. Bertolini, J. Betzwieser, D. Beveridge, N. Bevins, R. Bhandare, U. Bhardwaj, R. Bhatt, D. Bhattacharjee, S. Bhaumik, S. Bhowmick, A. Bianchi, I. A. Bilenko, G. Billingsley, A. Binetti, S. Bini, O. Birnholtz, S. Biscoveanu, A. Bisht, M. Bitossi, M. -A. Bizouard, J. K. Blackburn, L. A. Blagg, C. D. Blair, D. G. Blair, F. Bobba, N. Bode, G. Boileau, M. Boldrini, G. N. Bolingbroke, A. Bolliand, L. D. Bonavena, R. Bondarescu, F. Bondu, E. Bonilla, M. S. Bonilla, A. Bonino, R. Bonnand, P. Booker, A. Borchers, V. Boschi, S. Bose, V. Bossilkov, V. Boudart, A. Boudon, A. Bozzi, C. Bradaschia, P. R. Brady, M. Braglia, A. Branch, M. Branchesi, J. Brandt, I. Braun, M. Breschi, T. Briant, A. Brillet, M. Brinkmann, P. Brockill, E. Brockmueller, A. F. Brooks, B. C. Brown, D. D. Brown, M. L. Brozzetti, S. Brunett, G. Bruno, R. Bruntz, J. Bryant, F. Bucci, J. Buchanan, O. Bulashenko, T. Bulik, H. J. Bulten, A. Buonanno, K. Burtnyk, R. Buscicchio, D. Buskulic, C. Buy, R. L. Byer, G. S. Cabourn Davies, G. Cabras, R. Cabrita, V. Cáceres-Barbosa, L. Cadonati, G. Cagnoli, C. Cahillane, J. Calderón Bustillo, T. A. Callister, E. Calloni, J. B. Camp, M. Canepa, G. Caneva Santoro, K. C. Cannon, H. Cao, L. A. Capistran, E. Capocasa, E. Capote, G. Carapella, F. Carbognani, M. Carlassara, J. B. Carlin, M. Carpinelli, G. Carrillo, J. J. Carter, G. Carullo, J. Casanueva Diaz, C. Casentini, S. Y. Castro-Lucas, S. Caudill, M. CavagliÃ, R. Cavalieri, G. Cella, P. Cerdá-Durán, E. Cesarini, W. Chaibi, P. Chakraborty, S. Chalathadka Subrahmanya, J. C. L. Chan, M. Chan, K. Chandra, R. -J. Chang, S. Chao, P. Char, E. L. Charlton, P. Charlton, E. Chassande-Mottin, C. Chatterjee, Debarati Chatterjee, Deep Chatterjee, D. Chattopadhyay, M. Chaturvedi, S. Chaty, K. Chatziioannou, A. Chen, A. H. -Y. Chen, D. Chen, H. Chen, H. Y. Chen, J. Chen, K. H. Chen, Y. Chen, Yanbei Chen, Yitian Chen, H. P. Cheng, P. Chessa, H. T. Cheung, S. Y. Cheung, F. Chiadini, G. Chiarini, R. Chierici, A. Chincarini, M. L. Chiofalo, A. Chiummo, C. Chou, S. Choudhary, N. Christensen, S. S. Y. Chua, P. Chugh, G. Ciani, P. Ciecielag, M. Cieślar, M. Cifaldi, R. Ciolfi, F. Clara, J. A. Clark, J. Clarke, T. A. Clarke, P. Clearwater, S. Clesse, E. Coccia, E. Codazzo, P. -F. Cohadon, S. Colace, M. Colleoni, C. G. Collette, J. Collins, S. Colloms, A. Colombo, M. Colpi, C. M. Compton, G. Connolly, L. Conti, T. R. Corbitt, I. Cordero-Carrión, S. Corezzi, N. J. Cornish, A. Corsi, S. Cortese, C. A. Costa, R. Cottingham, M. W. Coughlin, A. Couineaux, J. -P. Coulon, S. T. Countryman, J. -F. Coupechoux, P. Couvares, D. M. Coward, M. J. Cowart, R. Coyne, K. Craig, R. Creed, J. D. E. Creighton, T. D. Creighton, P. Cremonese, A. W. Criswell, J. C. G. Crockett-Gray, S. Crook, R. Crouch, J. Csizmazia, J. R. Cudell, T. J. Cullen, A. Cumming, E. Cuoco, M. Cusinato, P. Dabadie, T. Dal Canton, S. Dall'Osso, S. Dal Pra, G. Dálya, B. D'Angelo, S. Danilishin, S. D'Antonio, K. Danzmann, K. E. Darroch, L. P. Dartez, A. Dasgupta, S. Datta, V. Dattilo, A. Daumas, N. Davari, I. Dave, A. Davenport, M. Davier, T. F. Davies, D. Davis, L. Davis, M. C. Davis, P. J. Davis, M. Dax, J. De Bolle, M. Deenadayalan, J. Degallaix, M. De Laurentis, S. Deléglise, F. De Lillo, D. Dell'Aquila, W. Del Pozzo, F. De Marco, F. De Matteis, V. D'Emilio, N. Demos, T. Dent, A. Depasse, N. DePergola, R. De Pietri, R. De Rosa, C. De Rossi, R. DeSalvo, R. De Simone, A. Dhani, R. Diab, M. C. Díaz, M. Di Cesare, G. Dideron, N. A. Didio, T. Dietrich, L. Di Fiore, C. Di Fronzo, M. Di Giovanni, T. Di Girolamo, D. Diksha, A. Di Michele, J. Ding, S. Di Pace, I. Di Palma, F. Di Renzo, Divyajyoti, A. Dmitriev, Z. Doctor, E. Dohmen, P. P. Doleva, D. Dominguez, L. D'Onofrio, F. Donovan, K. L. Dooley, T. Dooney, S. Doravari, O. Dorosh, M. Drago, J. C. Driggers, J. -G. Ducoin, L. Dunn, U. Dupletsa, D. D'Urso, H. Duval, P. -A. Duverne, S. E. Dwyer, C. Eassa, M. Ebersold, T. Eckhardt, G. Eddolls, B. Edelman, T. B. Edo, O. Edy, A. Effler, J. Eichholz, H. Einsle, M. Eisenmann, R. A. Eisenstein, A. Ejlli, R. M. Eleveld, M. Emma, K. Endo, A. J. Engl, E. Enloe, L. Errico, R. C. Essick, H. Estellés, D. Estevez, T. Etzel, M. Evans, T. Evstafyeva, B. E. Ewing, J. M. Ezquiaga, F. Fabrizi, F. Faedi, V. Fafone, S. Fairhurst, A. M. Farah, B. Farr, W. M. Farr, G. Favaro, M. Favata, M. Fays, M. Fazio, J. Feicht, M. M. Fejer, R. Felicetti, E. Fenyvesi, D. L. Ferguson, S. Ferraiuolo, I. Ferrante, T. A. Ferreira, F. Fidecaro, P. Figura, A. Fiori, I. Fiori, M. Fishbach, R. P. Fisher, R. Fittipaldi, V. Fiumara, R. Flaminio, S. M. Fleischer, L. S. Fleming, E. Floden, E. M. Foley, H. Fong, J. A. Font, B. Fornal, P. W. F. Forsyth, K. Franceschetti, N. Franchini, S. Frasca, F. Frasconi, A. Frattale Mascioli, Z. Frei, A. Freise, O. Freitas, R. Frey, W. Frischhertz, P. Fritschel, V. V. Frolov, G. G. Fronzé, M. Fuentes-Garcia, S. Fujii, T. Fujimori, P. Fulda, M. Fyffe, B. Gadre, J. R. Gair, S. Galaudage, V. Galdi, H. Gallagher, S. Gallardo, B. Gallego, R. Gamba, A. Gamboa, D. Ganapathy, A. Ganguly, B. Garaventa, J. García-Bellido, C. García Núñez, C. García-Quirós, J. W. Gardner, K. A. Gardner, J. Gargiulo, A. Garron, F. Garufi, C. Gasbarra, B. Gateley, V. Gayathri, G. Gemme, A. Gennai, V. Gennari, J. George, R. George, O. Gerberding, L. Gergely, S. Ghonge, Archisman Ghosh, Sayantan Ghosh, Shaon Ghosh, Shrobana Ghosh, Suprovo Ghosh, Tathagata Ghosh, L. Giacoppo, J. A. Giaime, K. D. Giardina, D. R. Gibson, D. T. Gibson, C. Gier, P. Giri, F. Gissi, S. Gkaitatzis, J. Glanzer, F. Glotin, J. Godfrey, P. Godwin, N. L. Goebbels, E. Goetz, J. Golomb, S. Gomez Lopez, B. Goncharov, Y. Gong, G. González, P. Goodarzi, S. Goode, A. W. Goodwin-Jones, M. Gosselin, A. S. Göttel, R. Gouaty, D. W. Gould, K. Govorkova, S. Goyal, B. Grace, A. Grado, V. Graham, A. E. Granados, M. Granata, V. Granata, S. Gras, P. Grassia, A. Gray, C. Gray, R. Gray, G. Greco, A. C. Green, S. M. Green, S. R. Green, A. M. Gretarsson, E. M. Gretarsson, D. Griffith, W. L. Griffiths, H. L. Griggs, G. Grignani, A. Grimaldi, C. Grimaud, H. Grote, D. Guerra, D. Guetta, G. M. Guidi, A. R. Guimaraes, H. K. Gulati, F. Gulminelli, A. M. Gunny, H. Guo, W. Guo, Y. Guo, Anchal Gupta, Anuradha Gupta, Ish Gupta, N. C. Gupta, P. Gupta, S. K. Gupta, T. Gupta, N. Gupte, J. Gurs, N. Gutierrez, F. Guzman, H. -Y. H, D. Haba, M. Haberland, S. Haino, E. D. Hall, E. Z. Hamilton, G. Hammond, W. -B. Han, M. Haney, J. Hanks, C. Hanna, M. D. Hannam, O. A. Hannuksela, A. G. Hanselman, H. Hansen, J. Hanson, R. Harada, A. R. Hardison, K. Haris, T. Harmark, J. Harms, G. M. Harry, I. W. Harry, J. Hart, B. Haskell, C. -J. Haster, J. S. Hathaway, K. Haughian, H. Hayakawa, K. Hayama, R. Hayes, A. Heffernan, A. Heidmann, M. C. Heintze, J. Heinze, J. Heinzel, H. Heitmann, F. Hellman, P. Hello, A. F. Helmling-Cornell, G. Hemming, O. Henderson-Sapir, M. Hendry, I. S. Heng, E. Hennes, C. Henshaw, T. Hertog, M. Heurs, A. L. Hewitt, J. Heyns, S. Higginbotham, S. Hild, S. Hill, Y. Himemoto, N. Hirata, C. Hirose, S. Hoang, S. Hochheim, D. Hofman, N. A. Holland, K. Holley-Bockelmann, Z. J. Holmes, D. E. Holz, L. Honet, C. Hong, J. Hornung, S. Hoshino, J. Hough, S. Hourihane, E. J. Howell, C. G. Hoy, C. A. Hrishikesh, H. -F. 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Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
For decades, astronomers have mapped the lives of stars, watching them burn bright and eventually collapse under their own weight. When a massive star dies, it leaves behind a dense core. If that core is heavy enough, it becomes a black hole, an object so dense that not even light can escape. If it is lighter, it becomes a neutron star, a city-sized sphere of matter so compressed that a single teaspoon would weigh billions of tons. For a long time, scientists believed there was a clear divide between these two families. They thought neutron stars topped out at a certain weight, and black holes started at a heavier weight, leaving a gap in between where no such objects could exist. This gap, spanning roughly three to five times the mass of our Sun, was a mystery that defined how we understood the death of stars.
In 2023, a team of scientists using a global network of gravitational wave detectors began to listen to the universe in a new way. These detectors, giant instruments built to sense ripples in space-time caused by violent cosmic events, finally caught a signal that challenged the old rules. The signal, named GW230529, came from a collision between two compact objects. One was a neutron star, and the other was a mysterious object with a mass that fell squarely into that forbidden gap. This discovery suggests that the gap between the heaviest neutron stars and the lightest black holes may not be empty, but rather contains objects that have been hiding in plain sight, although the region may still be less populated than the surrounding areas of the mass spectrum.
The event was detected on May 29, 2023, by the LIGO Livingston observatory in Louisiana. At that moment, the other detectors in the network were either offline or not sensitive enough to pick up the signal, meaning the discovery rested on the shoulders of a single instrument. Despite this limitation, the signal was clear and strong, appearing as a distinct chirp in the data that rose in frequency and intensity as the two objects spiraled toward each other. Three different computer programs, designed to sift through the noise of the universe to find these specific patterns, all identified the same event with high confidence. The data showed that the two objects were locked in a tight dance, pulling closer together until they merged into a single, more massive object.
When the researchers analyzed the data to determine the mass of the objects involved, they found a primary object weighing between 2.5 and 4.5 times the mass of our Sun. The second object was lighter, weighing between 1.2 and 2.0 solar masses. The lighter object fits perfectly with what we know about neutron stars. The heavier object, however, is the puzzle. It is too heavy to be a typical neutron star, yet too light to be a typical black hole. While the scientists cannot say with absolute certainty what the heavier object is, the most likely explanation is that it is a black hole that formed in the lower mass gap. This would make it the lightest black hole ever observed in a merging binary system.
The discovery is significant because it changes our understanding of how these objects form. If the heavier object is indeed a black hole, it suggests that the process of a star collapsing can sometimes produce objects in this intermediate weight range, rather than skipping straight to the heavier masses we usually see. The researchers also looked at the spin of the objects, which tells us how fast they were rotating. They found that the spins were not perfectly aligned with the orbit, a detail that helps refine models of how these binary systems come to be. While the data does not rule out the possibility that the heavier object is an unusually massive neutron star, the balance of evidence points toward a black hole.
This event also has implications for what happens when these objects collide. When a neutron star meets a black hole, the black hole can sometimes tear the neutron star apart before swallowing it, creating a flash of light and other radiation that telescopes can see. Because the two objects in this collision had masses that were more similar to each other than in previous observations, the chances of such a tidal disruption were higher. This means that future collisions of this type might be more likely to produce visible light, offering astronomers a chance to study these events with both gravitational wave detectors and traditional telescopes.
The team calculated that collisions like this one happen frequently enough to be a common occurrence in the universe, occurring roughly 55 times per cubic billion light-years every year. This rate suggests that the lower mass gap is not a rare anomaly but a standard part of the cosmic population. The discovery of GW230529 does not just add a new data point to a chart; it urges a change of paradigm in specific model assumptions regarding the formation of these objects. It confirms that the universe is more varied than previously thought, with objects existing in the spaces between our established categories. As the detectors continue to listen, they are likely to find more of these intermediate objects, filling in the gaps of our cosmic map and revealing the true diversity of how stars die.
Technical Summary: Observation of Gravitational Waves from the Coalescence of a 2.5–4.5 M⊙ Compact Object and a Neutron Star (GW230529)
Problem and Context
The paper addresses the uncertainty surrounding the mass distribution of compact objects, specifically the existence of a "lower mass gap" between the most massive neutron stars (NS) and the least massive black holes (BH). Previous dynamical mass measurements of Galactic X-ray binaries suggested a scarcity of compact objects between ∼3 and 5M⊙. While gravitational wave (GW) observations have begun to probe this region (e.g., GW190814), the population statistics remain sparse. The authors report the detection of a new compact binary coalescence (CBC) signal, GW230529, observed during the fourth observing run (O4) of the LIGO–Virgo–KAGRA (LVK) network, to further constrain the mass distribution and the nature of objects within this gap.
Methodology
- Detection: The signal was detected on May 29, 2023, at 18:15:00 UTC. Due to the offline status of LIGO Hanford, Virgo, and the limited sensitivity of KAGRA at the time, the analysis relied exclusively on data from the LIGO Livingston observatory. The signal was identified independently by three matched-filter search pipelines: GstLAL, MBTA, and PyCBC.
- Parameter Estimation: Source properties were inferred using Bayesian inference with the Bilby and Parallel Bilby libraries. The analysis utilized a 128-second data segment containing the signal. To mitigate systematic errors from waveform modeling, particularly for potential neutron star–black hole (NSBH) systems, the authors combined posterior samples from two distinct waveform models: the frequency-domain phenomenological model IMRPhenomXPHM and the time-domain effective-one-body model SEOBNRv5PHM. These models included higher-order multipole moments and spin-induced orbital precession but initially excluded tidal effects for the primary analysis.
- Population Analysis: The authors employed hierarchical Bayesian inference to update merger rate estimates and population models. They utilized three population models: the "NSBH-pop" model (parametric NSBH specific), the "Power law + Dip + Break" model (designed to detect mass gaps), and a nonparametric "Binned Gaussian Process" model.
- Nature Classification: To determine the nature of the components, the authors marginalized over uncertainties in mass, spin, redshift, and the equation of state (EOS) of supranuclear matter. They calculated the posterior probability that each component's mass and spin were consistent with the maximum limits supported by the EOS.
Key Results
- Source Properties: The binary consists of two components with masses m1=3.6−1.2+0.8M⊙ and m2=1.4−0.2+0.6M⊙ (90% credible intervals). The mass ratio is q=0.39−0.12+0.41. The primary mass is less than 5M⊙ at 99% credibility.
- Spin: The effective inspiral spin is measured as χeff=−0.10−0.17+0.12, consistent with a system where one spin component is antialigned with the orbital angular momentum or with negligible spins. No evidence for precession was found.
- Nature of Components:
- The secondary component is consistent with a neutron star (P(m2 is NS)≈96% under high-spin priors).
- The primary component resides in the lower mass gap (3–5M⊙). Under default high-spin priors, the probability that the primary is a neutron star is low (∼2.9%), favoring a black hole interpretation. However, when incorporating the "Power law + Dip + Break" population prior, the probability that the primary is a neutron star increases to ∼8.8% (and up to ∼27% if spin constraints are relaxed).
- Tidal deformability constraints were inconclusive; the data did not strongly favor or rule out tidal effects, consistent with the moderate signal-to-noise ratio.
- Merger Rates: The inclusion of GW230529 updates the estimated NSBH merger rate. Using an event-based approach, the rate for GW230529-like systems is estimated at 55−47+127 Gpc−3 yr−1. The population-based NSBH rate is updated to 94−64+109 Gpc−3 yr−1.
- Multimessenger Implications: Due to the relatively symmetric mass ratio compared to previous NSBH detections, GW230529 has a higher probability of tidal disruption. The authors estimate the probability of tidal disruption to be 0.1 (under high-spin priors), implying a higher likelihood of electromagnetic (EM) counterparts for similar systems. The fraction of NSBH mergers expected to be EM-bright increases from ≤0.06 to ≤0.18 with the inclusion of this event.
Significance and Claims
The paper claims that GW230529 provides further evidence for the existence of compact objects within the purported lower mass gap (3–5M⊙). While the authors cannot definitively determine the nature of the primary component from GW data alone, the most probable interpretation is the coalescence of a neutron star with a black hole having a mass between the heaviest known neutron stars and the lightest known black holes.
Key implications highlighted by the authors include:
- Mass Gap Occupation: The detection reinforces that the 3–5M⊙ range is not empty, though it may be less populated than surrounding regions.
- Symmetric NSBHs: GW230529 is the most symmetric-mass NSBH merger observed to date. This symmetry increases the expected rate of NSBH mergers that undergo tidal disruption and produce EM counterparts, potentially altering the predicted rate of EM-bright NSBH events.
- Formation Channels: The low mass of the primary challenges standard core-collapse supernova models, suggesting formation via fallback or accretion-induced collapse, or potentially as a merger remnant of two neutron stars (though the latter is deemed less probable due to rate constraints).
- Population Constraints: The event serves as a critical data point for refining population models of compact binaries, particularly regarding the mass ratio distribution and the shape of the mass spectrum near the lower mass gap.
The authors conclude that regardless of whether the primary is a black hole or a neutron star, GW230529 represents a novel addition to the CBC population and underscores the importance of continued exploration of the CBC parameter space in O4 and beyond.
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