Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries
The existence of a mass gap of 3–5 M _⊙ between the heaviest neutron stars (NSs) and the lightest black holes (BHs), inferred from the BH mass distribution in low-mass X-ray binaries (LMXBs), has been suggested for decades. The gravitational-wave (GW) source GW230529 is most likely a neutron star–bl...
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2025-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/adec6d |
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| author | Zepei Xing Vicky Kalogera Tassos Fragos Jeff J. Andrews Simone S. Bavera Max Briel Seth Gossage Konstantinos Kovlakas Matthias U. Kruckow Kyle Akira Rocha Meng Sun Philipp M. Srivastava Emmanouil Zapartas |
| author_facet | Zepei Xing Vicky Kalogera Tassos Fragos Jeff J. Andrews Simone S. Bavera Max Briel Seth Gossage Konstantinos Kovlakas Matthias U. Kruckow Kyle Akira Rocha Meng Sun Philipp M. Srivastava Emmanouil Zapartas |
| author_sort | Zepei Xing |
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| description | The existence of a mass gap of 3–5 M _⊙ between the heaviest neutron stars (NSs) and the lightest black holes (BHs), inferred from the BH mass distribution in low-mass X-ray binaries (LMXBs), has been suggested for decades. The gravitational-wave (GW) source GW230529 is most likely a neutron star–black hole (NSBH) merger, with the BH mass falling within this gap. This detection strongly challenges the existence of the gap and has implications for the NSBH population, including a revised BH mass distribution and an updated local merger rate. In this study, we employ POSYDON , a binary population synthesis code that integrates detailed single- and binary-star models, to investigate coalescing NSBH binaries, focusing on the BH mass distribution of the intrinsic NSBH merger population. For typical population models, we find that better matching the observed BH mass distribution requires the use of common-envelope (CE) efficiencies exceeding unity, a rather uncomfortable choice since most energy sources are already included. Alternatively, we find that a two-stage CE prescription calibrated to 1D hydrodynamic simulations has a similar effect. Moreover, motivated by a possible explanation for the firm presence of the mass gap in the LMXB sample, we examine models where the NS birth mass is limited to ≲2 M _⊙ and find excellent agreement with GW observations. Additionally, we present observable distributions of NSBH merger properties, finding good agreement with the measured properties of observed systems and a predicted fraction of NSBH mergers with potential electromagnetic counterparts that ranges from 1% all the way up to 32%. |
| format | Article |
| id | doaj-art-85de0645900f4c55bb7d1b80fea9f166 |
| institution | DOAJ |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj-art-85de0645900f4c55bb7d1b80fea9f1662025-08-20T03:02:52ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01989218810.3847/1538-4357/adec6dMass-gap Black Holes in Coalescing Neutron Star–Black Hole BinariesZepei Xing0https://orcid.org/0000-0002-0031-3029Vicky Kalogera1https://orcid.org/0000-0001-9236-5469Tassos Fragos2https://orcid.org/0000-0003-1474-1523Jeff J. Andrews3https://orcid.org/0000-0001-5261-3923Simone S. Bavera4https://orcid.org/0000-0002-3439-0321Max Briel5https://orcid.org/0000-0002-6842-3021Seth Gossage6https://orcid.org/0000-0001-6692-6410Konstantinos Kovlakas7https://orcid.org/0000-0003-3684-964XMatthias U. Kruckow8https://orcid.org/0000-0001-9331-0400Kyle Akira Rocha9https://orcid.org/0000-0003-4474-6528Meng Sun10https://orcid.org/0000-0001-9037-6180Philipp M. Srivastava11https://orcid.org/0000-0003-1749-6295Emmanouil Zapartas12https://orcid.org/0000-0002-7464-498XDépartement d’Astronomie, Université de Genève , Chemin Pegasi 51, CH-1290 Versoix, Switzerland ; Zepei.Xing@unige.ch; Gravitational Wave Science Center (GWSC), Université de Genève , CH-1211 Geneva, Switzerland; Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) , 1800 Sherman, Evanston, IL 60201, USACenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) , 1800 Sherman, Evanston, IL 60201, USA; Department of Physics & Astronomy, Northwestern University , 2145 Sheridan Road, Evanston, IL 60208, USADépartement d’Astronomie, Université de Genève , Chemin Pegasi 51, CH-1290 Versoix, Switzerland ; Zepei.Xing@unige.ch; Gravitational Wave Science Center (GWSC), Université de Genève , CH-1211 Geneva, SwitzerlandDepartment of Physics, University of Florida , 2001 Museum Road, Gainesville, FL 32611, USADépartement d’Astronomie, Université de Genève , Chemin Pegasi 51, CH-1290 Versoix, Switzerland ; Zepei.Xing@unige.ch; Gravitational Wave Science Center (GWSC), Université de Genève , CH-1211 Geneva, SwitzerlandDépartement d’Astronomie, Université de Genève , Chemin Pegasi 51, CH-1290 Versoix, Switzerland ; Zepei.Xing@unige.ch; Gravitational Wave Science Center (GWSC), Université de Genève , CH-1211 Geneva, SwitzerlandCenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) , 1800 Sherman, Evanston, IL 60201, USAInstitute of Space Sciences (ICE, CSIC) , Campus UAB, Carrer de Magrans, 08193 Barcelona, Spain; Institut d’Estudis Espacials de Catalunya (IEEC) , Carrer Gran Capità, 08034 Barcelona, SpainDépartement d’Astronomie, Université de Genève , Chemin Pegasi 51, CH-1290 Versoix, Switzerland ; Zepei.Xing@unige.ch; Gravitational Wave Science Center (GWSC), Université de Genève , CH-1211 Geneva, SwitzerlandCenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) , 1800 Sherman, Evanston, IL 60201, USA; Department of Physics & Astronomy, Northwestern University , 2145 Sheridan Road, Evanston, IL 60208, USACenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) , 1800 Sherman, Evanston, IL 60201, USACenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) , 1800 Sherman, Evanston, IL 60201, USA; Electrical and Computer Engineering, Northwestern University , 2145 Sheridan Road, Evanston, IL 60208, USAInstitute of Astrophysics , FORTH, N. Plastira 100, Heraklion, 70013, GreeceThe existence of a mass gap of 3–5 M _⊙ between the heaviest neutron stars (NSs) and the lightest black holes (BHs), inferred from the BH mass distribution in low-mass X-ray binaries (LMXBs), has been suggested for decades. The gravitational-wave (GW) source GW230529 is most likely a neutron star–black hole (NSBH) merger, with the BH mass falling within this gap. This detection strongly challenges the existence of the gap and has implications for the NSBH population, including a revised BH mass distribution and an updated local merger rate. In this study, we employ POSYDON , a binary population synthesis code that integrates detailed single- and binary-star models, to investigate coalescing NSBH binaries, focusing on the BH mass distribution of the intrinsic NSBH merger population. For typical population models, we find that better matching the observed BH mass distribution requires the use of common-envelope (CE) efficiencies exceeding unity, a rather uncomfortable choice since most energy sources are already included. Alternatively, we find that a two-stage CE prescription calibrated to 1D hydrodynamic simulations has a similar effect. Moreover, motivated by a possible explanation for the firm presence of the mass gap in the LMXB sample, we examine models where the NS birth mass is limited to ≲2 M _⊙ and find excellent agreement with GW observations. Additionally, we present observable distributions of NSBH merger properties, finding good agreement with the measured properties of observed systems and a predicted fraction of NSBH mergers with potential electromagnetic counterparts that ranges from 1% all the way up to 32%.https://doi.org/10.3847/1538-4357/adec6dGravitational wavesNeutron starsBlack holes |
| spellingShingle | Zepei Xing Vicky Kalogera Tassos Fragos Jeff J. Andrews Simone S. Bavera Max Briel Seth Gossage Konstantinos Kovlakas Matthias U. Kruckow Kyle Akira Rocha Meng Sun Philipp M. Srivastava Emmanouil Zapartas Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries The Astrophysical Journal Gravitational waves Neutron stars Black holes |
| title | Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries |
| title_full | Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries |
| title_fullStr | Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries |
| title_full_unstemmed | Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries |
| title_short | Mass-gap Black Holes in Coalescing Neutron Star–Black Hole Binaries |
| title_sort | mass gap black holes in coalescing neutron star black hole binaries |
| topic | Gravitational waves Neutron stars Black holes |
| url | https://doi.org/10.3847/1538-4357/adec6d |
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