Black hole–neutron star mergers: The first mass gap and kilonovae

Abstract

Observations of X-ray binaries indicate a dearth of compact objects in the mass range from ∼ 2 − 5 M . The existence of this (first mass) gap has been used to discriminate between proposed engines behind core-collapse supernovae. From LIGO/Virgo observations of binary compact remnant masses, several candidate first mass gap objects (either neutron stars (NSs) or black holes (BHs)) were identified during the O3 science run. Motivated by these new observations, we study the formation of BH-NS mergers in the framework of isolated classical binary evolution, using population synthesis methods to evolve large populations of binary stars (Population I and II) across cosmic time. We present results on the NS to BH mass ratios (q = M NS /M BH ) in merging systems, showing that although systems with a mass ratio as low as q = 0.02 can exist, typically BH-NS systems form with moderate mass ratios q = 0.1 − 0.2. If we adopt a delayed supernova engine, we conclude that ∼ 30% of BH-NS mergers may host at least one compact object in the first mass gap (FMG • ). Even allowing for uncertainties in the processes behind compact object formation, we expect the fraction of BH-NS systems ejecting mass during the merger to be small (from ∼ 0.6 − 9%). In our reference model, we assume: (i) the formation of compact objects within the FMG, (ii) natal NS/BH kicks decreased by fallback, (iii) low BH spins due to Tayler-Spruit angular momentum transport in massive stars. We find that 1% of BH-NS mergers will have any mass ejection and about the same percentage will produce kilonova bright enough to have a chance of being detected with a large (Subaru-class) 8m telescope. Interestingly, all these mergers will have both a BH and an NS in the FMG.

Can Neutron Star Mergers Alone Explain the r-process Enrichment of the Milky Way?

“…The sharp increase in Eu for supersolar metallicities is also problematic. Note that the value of f spin estimated in BPS studies is much smaller (Drozda et al 2022). The SN Ia model is the same as in K20.…”

“…Given the uncertain mass and spin distribution of BHs in merging BH-NS systems (e.g., Broekgaarden et al 2021), it is unclear whether a significant fraction of such mergers could contribute to r-process nucleosynthesis. Thus, while some short gamma-ray bursts and perhaps kilonovae have been conjectured to be associated with BH-NS mergers (Troja et al 2008;Berger 2014;Li et al 2017;Gompertz et al 2020), other authors have argued that the contribution of BH-NS mergers to short gamma-ray bursts is likely low (e.g., Drozda et al 2022). For example, it is probable that neither of the gravitational-wave observed BH-NS mergers would have experienced a kilonova (Abbott et al 2021a).…”

“…In addition, two mergers of an NS with a black hole (BH) have been confidently detected, GW200105 and GW200115 (Abbott et al 2021a); the provenance of GW190814, which could be an NS-BH or BH-BH merger, is unclear (Abbott et al 2020b). The mass ejection from these BH-NS mergers is significant only for a small fraction of events (perhaps 10%; Drozda et al 2022). There are simulations of BH-NS mergers (e.g., Janka et al 1999;Rosswog 2005), but estimating the total mass ejection from BH-NS mergers requires 3D general relativistic (GR) hydrodynamical simulations with neutrino transport Siegel & Metzger 2018; see also Foucart et al 2017 and the references therein).…”

Kilonovae and Optical Afterglows from Binary Neutron Star Mergers. II. Optimal Search Strategy for Serendipitous Observations and Target-of-opportunity Observations of Gravitational Wave Triggers

“…Kong et al 2023, in preparation), the Large Synoptic Survey Telescope (LSST; LSST Science Collaboration et al 2009), and the SiTian Projects (SiTian; Liu et al 2021). We note that (1) NSBH mergers may have a lower event rate density; (2) NSBH kilonovae may be dimmer than BNS kilonovae (e.g., Zhu et al 2020); (3) most NSBH mergers in the universe are likely plunging events (e.g., Zappa et al 2019;Drozda et al 2022;Abbott et al 2021a;Broekgaarden et al 2021;Zhu et al 2021cZhu et al , 2022bHu et al 2022). As a result, the detection rates of kilonova and afterglow emissions from NSBH mergers should be much lower than those from BNS mergers (Zhu et al 2021d).…”

Studying Postmerger Outflows from Magnetized-neutrino-cooled Accretion Disks

“…However, population synthesis studies of merger rates of BHNSs suggest that the majority of binaries will not result in observable EM counterparts. Studies by Drozda et al (2022) found that only a fraction (∼20%) of BHNS binaries gain a high dimensionless BH spin from their stellar progenitors and produce massive ejecta during the merger.…”