Mergers of black hole–neutron star binaries and rates of associated electromagnetic counterparts

The coalescence of double neutron star (NS–NS) and black hole (BH)–NS binaries are prime sources of gravitational waves (GW) for Advanced LIGO/Virgo and future ground-based detectors. Neutron-rich matter released from such events undergoes rapid neutron capture (r-process) nucleosynthesis as it decompresses into space, enriching our universe with rare heavy elements like gold and platinum. Radioactive decay of these unstable nuclei powers a rapidly evolving, approximately isotropic thermal transient known as a “kilonova”, which probes the physical conditions during the merger and its aftermath. Here I review the history and physics of kilonovae, leading to the current paradigm of day-timescale emission at optical wavelengths from lanthanide-free components of the ejecta, followed by week-long emission with a spectral peak in the near-infrared (NIR). These theoretical predictions, as compiled in the original version of this review, were largely confirmed by the transient optical/NIR counterpart discovered to the first NS–NS merger, GW170817, discovered by LIGO/Virgo. Using a simple light curve model to illustrate the essential physical processes and their application to GW170817, I then introduce important variations about the standard picture which may be observable in future mergers. These include hour-long UV precursor emission, powered by the decay of free neutrons in the outermost ejecta layers or shock-heating of the ejecta by a delayed ultra-relativistic outflow; and enhancement of the luminosity from a long-lived central engine, such as an accreting BH or millisecond magnetar. Joint GW and kilonova observations of GW170817 and future events provide a new avenue to constrain the astrophysical origin of the r-process elements and the equation of state of dense nuclear matter.

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“…See, e.g., Bhattacharya et al. (2019) for further discussion of the diverse EM counterparts of BH–NS mergers.…”

Section: Observational Prospects and Strategiesmentioning

confidence: 99%

Kilonovae

Metzger¹

2019

Living Rev Relativ

436

360

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“…For f s,NSBH we also explore the affect of an additional constraint; that the disk mass from an NSBH merger must be above 0.075 M to ensure the jet can successfully break through the ejecta as hypothesised in e.g., Zappa et al [48]. We calculate f s,NSBH enforcing this constraint using relations for the ejecta and disk mass in Bhattacharya et al [49]. We find that the discrepancy from the additional constraint of a minimum disk mass creates a change in f s,NSBH smaller than due to the unknown equation of state and we therefore ignore this additional constraint in subsequent sections.…”

Section: A Jet-launching Fractionsmentioning

confidence: 99%

Linking the rates of neutron star binaries and short gamma-ray bursts

Sarin,

Lasky,

Vivanco

et al. 2022

Preprint

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Short gamma-ray bursts are believed to be produced by both binary neutron star (BNS) and neutron star-black hole (NSBH) mergers. We use current estimates for the BNS and NSBH merger rates to calculate the fraction of observable short gamma-ray bursts produced through each channel. This allows us to constrain merger rates of BNS to RBNS = 384 +431 −213 Gpc −3 yr −1 (90% credible interval), a 16% decrease in the rate uncertainties from the second LIGO-Virgo Gravitational-Wave Transient Catalog, GWTC-2. Assuming a top-hat emission profile with a large Lorentz factor, we constrain the average opening angle of gamma-ray burst jets produced in BNS mergers to ≈ 15 • . We also measure the fraction of BNS and NSBH mergers that produce an observable short gamma-ray burst to be 0.02 +0.02 −0.01 and 0.01 ± 0.01, respectively and find that 40% of BNS mergers launch jets (90% confidence). We forecast constraints for future gravitational-wave detections given different modelling assumptions, including the possibility that BNS and NSBH jets are different. With 24 BNS and 55 NSBH observations, expected within six months of the LIGO-Virgo-KAGRA network operating at design sensitivity, it will be possible to constrain the fraction of BNS and NSBH mergers that launch jets with 10% precision. Within a year of observations, we can determine whether the jets launched in NSBH mergers have a different structure than those launched in BNS mergers and rule out whether 80% of binary neutron star mergers launch jets. We discuss the implications of future constraints on understanding the physics of short gamma-ray bursts and binary evolution.

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“…Although observations of high-mass X-ray binaries with black holes suggest that such systems should exist, the formation rate of merging BH-NS binaries is not well-constrained. BH-NS binaries have not been observed in electromagnetic (EM) signals (see also Bhattacharya et al (2019)). Here again, the O1/O2 science runs from the LIGO/Virgo consortium detected no BH-NS binaries.…”

Section: Introductionmentioning

confidence: 99%

Black hole - neutron star mergers: the first mass gap and kilonovae

Drozda¹,

Belczyński²,

O’Shaughnessy³

et al. 2020

Preprint

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Observations of X-ray binaries indicate a dearth of compact objects in the mass range from ∼ 2 − 5 M and the existence of this (first mass) gap has been used to advance our understanding of the engines behind core-collapse supernovae. LIGO/Virgo observations provide an independent measure of binary compact remnant masses and several candidate first mass gap objects (either NS or BH) were observed in the O3 science run. We study the formation of BH-NS mergers in the framework of isolated classical binary evolution. We use population synthesis method to evolve binary stars (Population I and II) across cosmic time. The predicted BH-NS mergers from the isolated classical binary evolution are sufficiently abundant (∼ 0.4 − 10 Gpc −3 yr −1 ) in the local Universe (z ≈ 0) to produce the observed LIGO/Virgo candidates. 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, only a small fraction (∼ 0.05% − 5%) of LIGO/Virgo detectable BH-NS mergers have mass ratios below q = 0.05. We find that with appropriate constraints on the (delayed) supernova engine ∼ 30 − 40% of LIGO/Virgo BH-NS mergers may host at least one compact object in the gap. The uncertainties in the processes behind compact object formation imply that the fraction of BH-NS systems ejecting mass during the merger is ∼ 0 − 9%. In our reference model where we assume: (i) formation of compact objects within the first mass gap, (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 only ∼ 0.2% of BH-NS mergers will have any mass ejection, and about the same percentage would produce kilonova bright enough to have a chance to be detected even with a large (Subaru-class) 8m telescope. Interestingly, all these mergers will have both BH and NS in the first mass gap.

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Mergers of black hole–neutron star binaries and rates of associated electromagnetic counterparts

Cited by 28 publications

References 91 publications

Kilonovae

Kilonovae

Linking the rates of neutron star binaries and short gamma-ray bursts

Black hole - neutron star mergers: the first mass gap and kilonovae

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