Multi-messenger prospects for black hole – neutron star mergers in the O4 and O5 runs^⋆

¹ Università degli Studi di Milano-Bicocca, Dip. di Fisica “G. Occhialini”, Piazza della Scienza 3, 20126 Milano, MI, Italy
e-mail: a.colombo140@campus.unimib.it
² INFN – Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, MI, Italy
³ INAF – Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, 23807 Merate, LC, Italy
⁴ Institut für Theoretische Physik, Goethe Universität Frankfurt am Main, 60323 Frankfurt am Main, Germany
⁵ Simons Society of Fellows, Simons Foundation, New York, NY 10010, USA
⁶ Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 550 W 120th St, New York, NY 10027, USA
⁷ William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁹ AstroAI at the Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁰ Département de Physique Théorique, Université de Genève, 24 quai Ernest Ansermet, 1211 Genève 4, Switzerland
¹¹ Gravitational Wave Science Center (GWSC), Université de Genève, 24 quai E. Ansermet, 1211 Geneva, Switzerland
¹² Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
¹³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA
¹⁵ SISSA, Via Bonomea 265, 34136 Trieste, Italy
¹⁶ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, RM, Italy
¹⁷ Space Science Data Center – Agenzia Spaziale Italiana, Via del Politecnico, s.n.c., 00133 Roma, Italy
¹⁸ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁹ Department of Astrophysics/IMAPP, Radboud University, 6525 AJ Nijmegen, The Netherlands
²⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK

Received: 25 October 2023
Accepted: 10 March 2024

Abstract

The existence of merging black hole-neutron star (BHNS) binaries has been ascertained through the observation of their gravitational wave (GW) signals. However, to date, no definitive electromagnetic (EM) emission has been confidently associated with these mergers. Such an association could help unravel crucial information on these systems, for example, their BH spin distribution, the equation of state (EoS) of the neutron star and the rate of heavy element production. We modeled the multi-messenger (MM) emission from BHNS mergers detectable during the fourth (O4) and fifth (O5) observing runs of the LIGO-Virgo-KAGRA (LVK) GW detector network in order to provide detailed predictions that can help enhance the effectiveness of observational efforts and extract the highest possible scientific information from such remarkable events. Our methodology is based on a population synthesis approach, which includes the modeling of the signal-to-noise ratio of the GW signal in the detectors, the GW-inferred sky localization of the source, the kilonova (KN) optical and near-infrared light curves, the relativistic jet gamma-ray burst (GRB) prompt emission peak photon flux, and the GRB afterglow light curves in the radio, optical, and X-ray bands. The resulting prospects for BHNS MM detections during O4 are not promising, with an LVK GW detection rate of 15.0_−8.8^+15.4 yr⁻¹, but joint MM rates of ∼10⁻¹ yr⁻¹ for the KN and ∼10⁻² yr⁻¹ for the jet-related emission. In O5, we found an overall increase in expected detection rates by around an order of magnitude, owing to both the enhanced sensitivity of the GW detector network and the coming online of future EM facilities. Considering variations in the NS EoS and BH spin distribution, we find that the detection rates can increase further by up to a factor of several tens. Finally, we discuss direct searches for the GRB radio afterglow with large field-of-view instruments during O5 and beyond as a new possible follow-up strategy in the context of ever-dimming prospects for KN detection due to the recession of the GW horizon.