Limits on the ejecta mass during the search for kilonovae associated with neutron star-black hole mergers: A case study of S230518h, GW230529, S230627c and the low-significance candidate S240422ed

Pillas, M.; Antier, S.; Ackley, K.; Ahumada, T.; Akl, D.; de Almeida, L.; Anand, S.; Andrade, C.; Andreoni, I.; Bostroem, K. A.; Bulla, M.; Burns, E.; Cabrera, T.; Chang, S.; Choi, H.; O'Connor, B.; Coughlin, M. W.; Corradi, W.; Gibbs, A. R.; Dietrich, T.; Dornic, D.; Ducoin, J.-G.; Duverne, P.-A.; Eggenstein, H.-B.; Freeberg, M.; Dyer, M.; Fausnaugh, M.; Fong, Wen-fai; Foucart, F.; Frostig, D.; Guessoum, N.; Gupta, Vaidehi; Hello, P.; Hosseinzadeh, G.; Hu, L.; Hussenot-Desenonges, T.; Im, M.; Jayaraman, R.; Jeong, M.; Karambelkar, V.; Kasliwal, M.; Kim, S.; Kilpatrick, C. D.; Kochiashvili, N.; Karpov, S.; Kunnumkai, K.; Lamoureux, M.; Lee, C. U.; Lourie, N.; Lyman, J.; Mašek, M.; Magnani, F.; Mo, G.; Molham, M.; Nitz, A. H.; Nicholl, M.; Navarete, F.; Noysena, K.; O'Neill, D.; Paek, G. S. H.; Palmese, A.; Poggiani, R.; Pradier, T.; Pyshna, O.; Rajabov, Y.; Rastinejad, J. C.; Sand, D. J.; Shawhan, P.; Shrestha, M.; Simcoe, R.; Smartt, S. J.; Steeghs, D.; Stein, R.; Stevance, H. F.; Takey, A.; Sun, M.; Toivonen, A.; Turpin, D.; Ulaczyk, K.; Wold, A.; Wouters, T.

doi:10.1103/6ld6-95xh

Published October 1, 2025 | Version Published

Journal Article Open

Limits on the ejecta mass during the search for kilonovae associated with neutron star-black hole mergers: A case study of S230518h, GW230529, S230627c and the low-significance candidate S240422ed

1. Université Côte d'Azur
2. University of Liège
3. University of Paris-Saclay
4. University of Warwick
5. California Institute of Technology
6. American University of Sharjah
7. Laboratório Nacional de Astrofísica
8. University of Minnesota
9. University of North Carolina at Chapel Hill
10. University of Arizona
11. University of Ferrara
12. INFN Sezione di Ferrara
13. Collurania Teramo Observatory
14. Louisiana State University
15. Carnegie Mellon University
16. Seoul National University
17. University of Potsdam
18. Max Planck Institute for Gravitational Physics
19. Aix-Marseille University
20. University of Paris
21. University of Sheffield
22. Massachusetts Institute of Technology
23. Northwestern University
24. University of New Hampshire
25. Harvard-Smithsonian Center for Astrophysics
26. Laboratoire de Physique des 2 Infinis Irène Joliot-Curie
27. University of California, San Diego
28. Abastumani Astrophysical Observatory
29. Institute of Physics
30. Université Catholique de Louvain
31. Korea Astronomy and Space Science Institute
32. National Research Institute of Astronomy and Geophysics
33. Syracuse University
34. Queen's University Belfast
35. NOIRLab
36. National Astronomical Research Institute of Thailand
37. University of Hawaii at Manoa
38. University of Pisa
39. University of Strasbourg
40. Taras Shevchenko National University of Kyiv
41. Ulugh Beg Astronomical Institute
42. University of Maryland, College Park
43. University of Oxford
44. Joint Space Science Institute
45. Goddard Space Flight Center
46. Utrecht University
47. National Institute for Subatomic Physics

Neutron star-black hole (NSBH) mergers, detectable via their gravitational-wave (GW) emission, are expected to produce kilonovae (KNe). Four NSBH candidates have been identified and followed-up by more than fifty instruments since the start of the fourth GW observing run (O4), in May 2023, up to July 2024; however, no confirmed associated KN has been detected. This study evaluates ejecta properties from multimessenger observations to understand the absence of detectable KN: we use GW public information and joint observations taken from 05.2023 to 07.2024 (LVK, ATLAS, DECam, GECKO, GOTO, GRANDMA, SAGUARO, TESS, WINTER, ZTF). First, our analysis on follow-up observation strategies shows that, on average, more than 50% of the simulated KNe associated with NSBH mergers reach their peak luminosity around one day after merger in the 𝑔, 𝑟, 𝑖- bands, which is not necessarily covered for each NSBH GW candidate. We also analyze the trade-off between observation efficiency and the intrinsic properties of the KN emission, to understand the impact on how these constraints affect our ability to detect the KN, and underlying ejecta properties for each GW candidate. In particular, we can only confirm the kilonova was not missed for 1% of the GW230529 and S230627c sky localization region, given the large sky localization error of GW230529 and the large distance for S230627c and, their respective KN faint luminosities. More constraining, for S230518h, we infer the dynamical ejecta and postmerger disk wind ejecta 𝑚dyn, 𝑚wind < 0.03⁢𝑀⊙ and the viewing angle 𝜃 >25°. Similarly, the nonastrophysical origin of S240422ed is likely further confirmed by the fact that we would have detected even a faint KN at the time and presumed distance of the S240422ed event candidate, within a minimum 45% credible region of the sky area, that can be larger depending on the KN scenario.

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Acknowledgement

A. H. N. acknowledges support from NSF Grant No. PHY-2309240. M. W.C, C. A. and V. G. acknowledge support from the National Science Foundation with Grants No. PHY-2308862 and No. PHY-2117997. The authors of M. M. and S. K. thanks to the following grants of the Ministry of Education of the Czech Republic LM2023032 and LM2023047, as well as EU/MEYS grants CZ.02.1.01/0.0/0.0/16_013/0001403, CZ.02.1.01/0.0/0.0/18_046/0016007, CZ.02.1.01/0.0/0.0/ 16_019/0000754, and CZ.02.01.01/00/22_008/0004632. S. A. thanks the CNRS MITI and TAROT Collaboration for providing support for this work. M. L. is a Postdoctoral Researcher of the Fonds de la Recherche Scientifique—M. P. acknowledges support from FNRS and IISN 4.4503. T. W. acknowledges funding through NWO under Grant No. OCENW.XL21.XL21.038. The work of F. N. is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. A. P. is supported by NSF Grant No. 2308193. Based on observations at Cerro Tololo Inter-American Observatory, NSF’s NOIRLab (NOIRLab Prop. ID 2023B-851374, PI: Andreoni and Palmese), which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. W. C. acknowledges the support from CNPq—BRICS 440142/2022-9, FAPEMIG APQ 02493-22 and FNDCT/FINEP/REF 0180/22. KAO-NRIAG team acknowledges financial support from the Egyptian Science, Technology and Innovation Funding Authority (STDF) under Grant No. 45779. F. F. gratefully acknowledges support from the Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02-05CH11231 and from NASA through Grant No. 80NSSC22K0719. H. F. S. is supported by Schmidt Sciences. S. J. S. acknowledges funding from STFC Grants No. ST/Y001605/1, No. ST/X006506/1, No. ST/T000198/1, a Royal Society Research Professorship and the Hintze Charitable Foundation. We thank I. M. G. and K. C. for the discussion regarding the ejecta mass computation. S. A. and M. P. thank M. K. for contributions on expertise and data related to ZTF for this work. This publication was made possible through the support of Grant No. 62192 from the John Templeton Foundation to LSST-DA. We acknowledge funding from the Daimler and Benz Foundation for the project “NUMANJI” and from the European Union (ERC, SMArt, 101076369). Neither the European Union nor the granting authority can be held responsible for them. The Programme National des Hautes Énergies (PNHE) of CNRS/INSU cofunded by CNRS/IN2P3, CNRS/INP, CEA and CNES supported this work. N. K. thanks V. Ayvazyan and R. Inasaridze for contribution of AbAO observations during O4: results are published in GCNs. S. A. thanks les Makes Observatory, NOWT, ASTEP, UBAI-T60, OST-CDK, OPD-60, NOWT, TRT, Kilonova-catcher, FRAM, KAO, AbAO-T70, F. Rünger, P. Thierry, A. Klotz, A. Ishankar, R. Hainich, H. B. Eggesntein, P. Vignes, P. Jacquiery, T. Guillot, V. Deloupy, O. Burkhonov., E. Gurbanov, S. Ehgamberdiev, T. Sadibekova, I. Tosta e Melo, X. F. Wang, J. Zhu, X. Zeng, A. Iskandar, S. Pormente, P. Gokuldass, L. Abe, K. Agabi, J. Dibasso, for useful coordination and observation of the O4 GW candidates during O4, results are published in GCNs (GCN 34130, GCN 36284, 36299, 36326). M. M. and S. K. thank M. Prouza for providing useful contributions to the observation of FRAM of EM counterpart follow-up during the O4 campaign. S. A. thanks IJCLAB and the Skyportal team for the set up of a great system of ICARE, especially M. Jouvin, J. Peloton, N. Leroy, T. Du Laz. A. Takey thank E. G. Elhosseiny and M. Abdelkareem for providing useful contributions to the observation of KAO of EM counterpart of candidates during the O4 campaign. W. C., F. N., and T. de Almeida thank L. Fraga, N. Sasaki for their contribution to the observations of OPD-60 of EM counterpart of candidates during the O4 campaign. S. A. and M. P. thank Y. Tillayev, O. Burkhonov for their contribution to the observations of UBAI EM counterpart of candidates during the O4 campaign. S. A. and M. P. thank O. Sokoliuk for discussion to improve the quality of the paper. M. I., G. S. H. P., M. J., S. W. C., H. C., C. U. L., and S. K. acknowledge the support from the National Research Foundation of Korea (NRF) Grants No. 2020R1A2C3011091, and No. 2021M3F7A1084525, funded by the Korea government (MSIT). This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near Earth asteroids through NASA Grants No. NN12AR55G, No. 80NSSC18K0284, and No. 80NSSC18K1575; by-products of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 Grant No. J1944/80NSSC19K0112 and No. HST GO-15889, and STFC Grants No. ST/T000198/1 and No. ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. We thank the observers, especially Joh-Na Yoon, Hongjae Moon, Sumin Lee, Changgon Kim, Soojong Pak, and Jin-Guk Seo, of the Kyung Hee Astronomical Observatory, CBNUO, the SNU Astronomical Observatory, and KMTNet for performing the GECKO observations of several targets. This research has made use of the KMTNet system operated by the Korea Astronomy and Space Science Institute (KASI) at three host sites CTIO in Chile, SAAO in South Africa, and SSO in Australia. Data transfer from the host site to K. A. S. I. was supported by the Korea Research Environment Open NETwork (KREONET). D. Frostig’s contribution to this material is based upon work supported by the National Science Foundation under Award No. AST-2401779. Time-domain research by the University of Arizona team and D. J. S. is supported by National Science Foundation (NSF) Grants No. 2108032, No. 2308181, No. 2407566, and No. 2432036 and the Heising-Simons Foundation under Grant No. 2020-1864. This work was supported by Shota Rustaveli National Science Foundation of Georgia (SRNSFG) Grant No. FR-24-7713. M. P. and S. A. thanks the MOCpy’s team [143] and especially Manon Marchand and Francois-Xavier PINEAU for close communication to produce the skymap observations of the article. M. N. is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 948381) and by UK Space Agency Grant No. ST/Y000692/1. I. A. is supported by NASA Award No. 24-ADAP24-0159 and NSF Award No. 2407924. The authors gratefully acknowledge the support of the NSF, STFC, INFN and CNRS for provision of computational resources. This material is based upon work supported by NSF’s LIGO Laboratory which is a major facility fully funded by the National Science Foundation. K. A. Bostroem acknowledges support from LSST-DA Catalyst Fellowship.

Additional Information

The opinions expressed in this publication are those of the author(s) and do not necessarily reflect the views of LSST-DA or the John Templeton Foundation. Views and opinions expressed are those of the authors only and do not necessarily reflect those of the European Union or the European Research Council.

Additional Information

M. P. and S. A. are the main contributors to the work including the analysis and writing across all sections, coordination and organization of contributors related to observational and GW and high energy results. M. P. is the main author of Sec. II—2.2, Sec. IV and Appendix of IV, V., C, D and E. M. Pillas is the main author of Sec. IV with help of S. A., J.-G. D., T. H. and M. C. M. P. is the main author of Sec. V with contributions of S. A. P.-A. Duverne and Y. R. are co-contributors to Sec. I, with the coordination of S. A. and M. P. S. A. and P. H. are co-contributors of Sec. II—2.1 related to GW with the help of F. M. S. A. and F. F. are the main contributors of Sec. III-3.1, and 3.2 with crucial contribution of A. H. N. S. A. is the main contributor of Sec. A.1, A.3, A.4, and B with contributions from D. A., M. P., O. P., R. P., J.-G. D. and L. A. W. C., H.-B. E., M. F., S. K., N. K., M. M., M. M., A. T., K. N. contributed to this work via observations and measurements taken by GRANDMA observatory. C. A., E. B., W. C., T. D., T. H., D. D., N. G., P. H., M. L., F. N., T. P., P. S., M. S., A. T., D. T., T. W. contributed to the review and edition of the full article to improve the quality of the article. T. A., A. W., M. W. C., M. K. and S. A. contibruted to this work via observations and measurements taken by the Zwicky Transient Facility. I. A., A. P., T. C., L. H., K. K., B. O’Connor to this work via observations and measurements taken by the DECAM. In addition, V. Gupta contributed to Sec. II—2.3 and IV—4.1 by providing pointings of S240422ed followup from DECam telescope. K. A., M. D., D. O’Neill provided coverage and analysis information related to GOTO follow-up. K.Ulaczyk and J.Lyman were key to enable the data analysis. D.Steeghs contributed to manuscript review and provided additional text. M. I., G. P., M. J., S.-W. C., S. K., H. C., C. U. L. contributed to this work via observations and measurements taken by GECKO. D. F., V. K., M. K., N. L., G. M., R. S., and R. S. contributed to this work via observations and measurements taken by WINTER. M. N., S. J. S., H. F. S. contributed to this work via observations and measurements taken from ATLAS. K. A. B., W. F., A. R. G., G. H., C. D. K., J. R., D. J. S., M. S. contributed to this work via observations and measurements taken by SAGUARO and CSS.

Data Availability

The data that support the findings of this article are openly available [144].

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Is new version of: Discussion Paper: arXiv:2503.15422 (arXiv)
Is supplemented by: Dataset: https://github.com/MPillas/KN-NSBH (URL)

National Science Foundation
PHY-2309240
National Science Foundation
PHY-2308862
National Science Foundation
PHY-2117997
Ministry of Education Youth and Sports
LM2023032
Ministry of Education Youth and Sports
LM2023047
Ministry of Education Youth and Sports
CZ.02.1.01/0.0/0.0/16_013/0001403
Ministry of Education Youth and Sports
CZ.02.1.01/0.0/0.0/18_046/0016007
Ministry of Education Youth and Sports
CZ.02.1.01/0.0/0.0/ 16_019/0000754
Ministry of Education Youth and Sports
CZ.02.01.01/00/22_008/0004632
Centre National de la Recherche Scientifique
TAROT
Fund for Scientific Research
IISN
4.4503
Dutch Research Council
OCENW.XL21.XL21.038
National Science Foundation
2308193
NOIRLab
2023B-851374
National Council for Scientific and Technological Development
440142/2022-9
Fundação de Amparo à Pesquisa do Estado de Minas Gerais
APQ 02493-22
Financiadora de Estudos e Projetos
REF 0180/22
Egyptian Center for Innovation and Technology Development
45779
United States Department of Energy
DE-AC02-05CH11231
National Aeronautics and Space Administration
80NSSC22K0719
Schmidt Family Foundation
Science and Technology Facilities Council
ST/Y001605/1
Science and Technology Facilities Council
ST/X006506/1
Science and Technology Facilities Council
ST/T000198/1
Royal Society
Hintze Family Charitable Foundation
John Templeton Foundation
62192
Daimler and Benz Foundation
European Union
101076369
Institut National des Sciences de l'Univers
Istituto Nazionale di Fisica Nucleare
National Council for Scientific and Technological Development
Institut National de Physique Nucléaire et de Physique des Particules
Programme National des Hautes Énergies
Institut de Physique
Commissariat à l'Énergie Atomique et aux Énergies Alternatives
Centre National d'Études Spatiales
National Research Foundation of Korea
2020R1A2C3011091
National Research Foundation of Korea
2021M3F7A1084525
Ministry of Science and ICT
National Aeronautics and Space Administration
NN12AR55G
National Aeronautics and Space Administration
80NSSC18K0284
National Aeronautics and Space Administration
80NSSC18K1575
National Aeronautics and Space Administration
Kepler/K2 J1944/80NSSC19K0112
National Aeronautics and Space Administration
HST GO-15889
Science and Technology Facilities Council
ST/S006109/1
Korea Research Environment Open Network
National Science Foundation
AST-2401779
National Science Foundation
2108032
National Science Foundation
2308181
National Science Foundation
2407566
National Science Foundation
2432036
Heising-Simons Foundation
2020-1864
Shota Rustaveli National Science Foundation of Georgia
FR-24-7713
European Research Council
948381
United Kingdom Space Agency
ST/Y000692/1
National Aeronautics and Space Administration
24-ADAP24-0159
National Science Foundation
2407924

Accepted: 2025-07-18

Caltech groups: Astronomy Department, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status: Published

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Limits on the ejecta mass during the search for kilonovae associated with neutron star-black hole mergers: A case study of S230518h, GW230529, S230627c and the low-significance candidate S240422ed

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Limits on the ejecta mass during the search for kilonovae associated with neutron star-black hole mergers: A case study of S230518h, GW230529, S230627c and the low-significance candidate S240422ed

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