Prompt-Collapse Binary Neutron Star Mergers

Updated 29 July 2025

Prompt-collapse binary neutron star mergers are defined by a collapse to a black hole within a few milliseconds, offering critical constraints on the high-density equation of state.
Empirical relations show that the threshold mass scales with the maximum neutron star mass (k ~ 1.3–1.7), with mass ratio and compactness significantly influencing the collapse process.
Numerical relativity simulations indicate that these mergers generate high-frequency gravitational wave bursts (∼5 kHz) and yield negligible ejecta, limiting electromagnetic counterparts.

A prompt-collapse binary neutron star (BNS) merger is an astrophysical event in which two neutron stars coalesce such that dynamical collapse to a black hole occurs on a timescale comparable to or less than the dynamical timescale of the system (typically ≲ few milliseconds after contact). The dynamics, observable signatures, and theoretical understanding of prompt-collapse mergers critically inform the properties of ultra-dense matter and the astrophysical pathways to electromagnetic and gravitational-wave transients.

1. Collapse Thresholds and Equation of State Dependence

The threshold mass, $M_{\rm th}$ , for prompt collapse separates equal-mass BNS mergers that form hypermassive or supramassive neutron star remnants from those that directly produce a black hole. The value of $M_{\rm th}$ is not universal but is tightly governed by the high-density equation of state (EoS) and the stellar compactness (Bauswein et al., 2013, Bauswein et al., 2017, Köppel et al., 2019, Bauswein et al., 2020).

Scaling relations for the prompt-collapse threshold:

The threshold mass obeys $M_{\rm th} = k \cdot M_{\rm max}$ where $M_{\rm max}$ is the maximum gravitational mass of an isolated, nonrotating neutron star (the TOV limit) and $k$ is a coefficient set by the EoS stiffness and stellar compactness.
Simulations indicate $k \sim 1.3$ –$1.7$, with lower values for stiffer (larger-radius) EoSs and higher values for softer (more compact) EoSs (Bauswein et al., 2013).
$k$ correlates nearly linearly with the compactness $C_{\rm max} = G M_{\rm max}/(c^2 R_{\rm max})$ . Empirical relations take the form $k = a + b C_{\rm max}$ (fit parameters in (Bauswein et al., 2017)).
A nonlinear relation between $M_{\rm th}/M_{\rm TOV}$ and maximum compactness further refines the threshold determination, especially near black hole formation (Köppel et al., 2019).

For binaries with significant mass asymmetry ( $q \lesssim 0.85$ ), $M_{\rm th}$ generally decreases with decreasing $q$ , enhancing the tendency to prompt collapse (Bauswein et al., 2020, Perego et al., 2021, Kölsch et al., 2021).

Role of microphysics and exotic degrees of freedom:

Hyperonic matter or phase transitions to deconfined quarks soften the EoS at high density, reducing $M_{\rm th}$ by typically $\sim0.05\,M_\odot$ compared to nucleonic models of matched low-density properties (Tolos et al., 24 Jul 2025, Bauswein et al., 2020).
Inclusion of a massive scalar field (in scalar-tensor theories) raises $M_{\rm th}$ by $\sim0.1$ – $0.2\,M_\odot$ via additional effective pressure support in the remnant (Lam et al., 7 Jun 2024).

2. Binary and Remnant Parameters Governing the Outcome

Impact of Binary Mass and Inclination

For $M_{\rm tot} > M_{\rm th}$ , prompt collapse to black hole occurs within $\lesssim 2$ ms of merger (Kölsch et al., 2021, Schianchi et al., 26 Feb 2024).
The mass ratio $q = M_1/M_2$ modulates $M_{\rm th}$ : increasing asymmetry lowers the stability of the remnant and results in more frequent prompt collapses at a given $M_{\rm tot}$ (Bauswein et al., 2020, Kölsch et al., 2021).
Intrinsic spin aligned with the orbit increases centrifugal support, thus delaying collapse or inhibiting prompt collapse; anti-aligned spin accelerates it (Schianchi et al., 26 Feb 2024, Karakas et al., 22 May 2024). For highly spinning setups, prompt-collapse can yield black holes with dimensionless spin up to $\chi_{\rm BH} \sim 0.92$ (Karakas et al., 22 May 2024).

Remnant and Disk/Ejecta Properties

Scenario	Disk Mass	Ejecta Mass	BH Spin	Comments
Prompt collapse	$\ll 0.01\,M_\odot$	$\lesssim10^{-4} M_\odot$	$0.85$–$0.95$	Negligible disk/ejecta; rapid collapse
Delayed collapse (HMNS)	$0.01$– $0.1\,M_\odot$	$0.01$– $0.07\,M_\odot$	$0.6$–$0.85$	Substantial disk; more ejecta

In prompt-collapse events, nearly all material is accreted by the black hole; the rest mass left outside is typically $\leq 10^{-6}$ – $10^{-4}$ of the original (1208.5279, Kölsch et al., 2021, Dhani et al., 25 Jul 2025).
If the collapse is even minimally delayed (by, e.g., extra centrifugal support), a higher disk mass and more ejected material are possible. Increased mass ratio exacerbates this effect, as the less massive star is tidally disrupted prior to collapse, resulting in more remnant disk and ejecta (Kölsch et al., 2021).
The final mass and spin of the remnant black hole are remarkably constrained across prompt-collapse simulations: $a_{\rm f} \sim 0.85$ –$0.95$. The remnant angular momentum and radiated GW energy scale quadratically; prompt-collapse systems retain more mass and angular momentum than BBH systems of the same total mass (Zappa et al., 2017, Dhani et al., 25 Jul 2025).

3. Gravitational-Wave and Electromagnetic Diagnostics

Gravitational-Wave Observables

Prompt-collapse mergers yield a short, high-frequency GW burst at $\sim5\,$ kHz, corresponding to black-hole ringdown; if followed by a hypermassive neutron star, quasi-periodic GW emission at $2$– $4\,$ kHz is seen (1208.5279, Zappa et al., 2017, Tringali et al., 2023).
The GW peak luminosity is maximized in prompt collapse events, but the total radiated GW energy is smaller than in mergers forming a long-lived remnant (Zappa et al., 2017). An empirical quadratic relation links total GW energy and remnant angular momentum, placing the final BH spin in prompt-collapse in the range $0.75 \lesssim a_{\rm BH} \lesssim 0.8$ (Zappa et al., 2017).

Astrophysical Counterparts

Due to the vanishingly small ejecta and disk masses, canonical EM signals (bright kilonovae, SGRBs from jet formation) are strongly suppressed in prompt-collapse BNS mergers with $q\gtrsim0.8$ (Paschalidis et al., 2018, Ruiz et al., 2017). The rapid collapse prevents significant shock heating, mass ejection, and magnetic field amplification necessary for jet launching (Ruiz et al., 2017, Kölsch et al., 2021, Karakas et al., 22 May 2024).
Instead, pre-merger magnetospheric interactions and post-collapse relaxation of magnetospheric energy may yield ms-duration, non-repeating fast radio bursts (FRBs) with luminosities matching observed FRB energetics if even a small fraction ( $\sim1\%$ ) of $E_B \sim 10^{39}$ – $10^{41}$ erg is radiated after prompt collapse; this is supported by full GRMHD (magnetized) simulations (Paschalidis et al., 2018, Nathanail, 2020).
Observed multi-messenger events (such as GW170817) that produce kilonovae, sGRBs, and afterglows are interpreted as having avoided prompt collapse (i.e., sub-critical $M_{\rm tot}$ ); constraints on the EoS, maximum mass, and radius ensue from this inference (Li et al., 30 Apr 2024, Köppel et al., 2019).

4. Microphysics, Exotic Matter, and Implications for Dense Matter

In the presence of thermally produced hyperons, the threshold mass for prompt collapse is systematically reduced by $\sim0.05\,M_\odot$ ; this is directly attributable to the increased specific heat and lower thermal pressure in hyperonic matter. The dominant postmerger GW frequency increases by 2%–4% compared to nucleonic models, and temperature evolution is suppressed (Tolos et al., 24 Jul 2025).
Phase transitions to quark matter can sharply lower $M_{\rm th}$ while not significantly affecting tidal deformability before merger, leading to “peculiar” combinations in the observed GW event properties that could serve as a diagnostic for exotic matter (Bauswein et al., 2020).
Nuclear matter incompressibility at several times nuclear saturation density, $K_{\rm max}$ , controls the sensitivity of $M_{\rm th}$ to mass ratio and sets the threshold; GW observations of prompt collapse in binaries with different $q$ can be used to measure $K_{\rm max}$ within tens of percent, potentially revealing the presence of hyperons or quarks (Perego et al., 2021).

5. Numerical Relativity Modeling and Observational Prospects

Large-scale relativity simulations, incorporating various microphysics (finite-temperature EoSs, neutrino transport, magnetic fields), underpin all prompt-collapse BNS merger modeling (Bauswein et al., 2013, Kölsch et al., 2021, Karakas et al., 22 May 2024). Threshold masses are rigorously determined by tracking the collapse time as a function of $M_{\rm tot}$ ; $t_{\rm coll} \lesssim 2$ ms typically signifies prompt collapse (Kölsch et al., 2021, Schianchi et al., 26 Feb 2024).
Next-generation GW detectors (e.g., Cosmic Explorer) will be able to distinguish prompt-collapse BNS mergers from binary black hole systems of identical mass and spin by measuring finite-size (tidal) effects in the late inspiral. Reduced tidal deformabilities $\tilde\Lambda$ as small as $\sim3.5$ can be distinguished from zero at $100$ Mpc, exceeding the reach for direct postmerger detection, which typically achieves SNRs in the 4–8 range for optimally located sources at this distance (Dhani et al., 25 Jul 2025).
Even in cases where the postmerger GW signal is not visible, the inferred prompt-collapse outcome (from lack of significant EM emission or inspiral masses above the empirical threshold) can set strong constraints on the neutron star EoS, the maximum mass $M_{\rm max}$ , and the presence of new degrees of freedom (Bauswein et al., 2020, Köppel et al., 2019, Dhani et al., 25 Jul 2025).

6. Future Directions and Implications

Increasing simulation fidelity (higher resolution, improved microphysics, larger parameter spaces for EoS and spin) is expected to refine the modeling of prompt-collapse thresholds, precise disk/ejecta masses, and the GW/EM signals (Ruiz et al., 2017, Dhani et al., 25 Jul 2025).
Coordinated multimessenger searches—especially rapid radio, X-ray, and optical follow-up of GW-detected mergers—will be required to identify FRB-like signatures from orphan prompt-collapse events, constraining both EoS and merger rates (Paschalidis et al., 2018, Nathanail, 2020).
Observational constraints on $M_{\rm th}$ and compactness from future events will provide further input on the high-density EoS, possibly revealing or ruling out the existence of exotic states of matter (deconfined quarks, hyperons) in neutron star interiors (Tolos et al., 24 Jul 2025, Bauswein et al., 2020, Perego et al., 2021).
The nearly universal nature of remnant properties (high mass and spin with low disk and ejecta for prompt collapse) and their GW signatures place prompt-collapse BNS mergers as a unique laboratory for exploring matter at extreme density and gravity.