Missing Mergers: On the Unexpected Delay-Time Distribution of Binary Neutron Star Mergers
Abstract
The LIGO-Virgo-KAGRA (LVK) collaboration has detected two binary neutron star (BNS) mergers, GW170817 and GW190425. While no clear electromagnetic counterpart was discovered for GW190425, a coincident counterpart to GW170817 (GRB170817A, AT 2017gfo) was discovered in an early-type galaxy, NGC 4993. Using broad-band photometry and an integrated optical spectrum, we measure a mass-weighted age of 12.4+0.5−0.7 Gyr (90% credible) for NGC 4993, which is an estimate of the delay time between star formation and merger for the GW170817 progenitor. BNS systems are expected to have a power-law delay-time distribution (DTD) with a minimum delay time, tmin, of ~10 Myr and index α≈ −1. This canonical DTD, convolved with the cosmic star-formation history, predicts ~16 BNS mergers at delay times of ~100 Myr for every BNS merger at 10 Gyr; under it, the chance of detecting even one merger as old as GW170817 is ~1.8%. The long delay time of GW170817 therefore implies many "missing" shorter-delay mergers if the canonical DTD holds. A DTD with tmin = 12.6 Gyr and α= −1 is favored over the canonical DTD by a factor of 40-140, and over short gamma-ray burst and Galactic double-NS DTDs by factors of 80-280. These results are in tension with current population synthesis models, the Milky Way double NS population, and the SGRB population. Explaining the LVK population requires either a dramatically different separation distribution than commonly assumed, an additional channel with a long delay time and comparable rate to an isolated-binary formation scenario, or a large population of BNS mergers missed in the LVK analysis. This result has wide-ranging implications for binary-population synthesis, chemical enrichment models, connections between BNS mergers and SGRBs, and future gravitational wave observations.
Related
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- Charged-current neutrino opacity within the relativistic Hartree-Fock framework for astrophysical simulations of core-collapse supernovae and binary neutron star mergers
- Nuclear Physics of Binary Neutron Star Mergers
BibTeX
@misc{foley2026missing,
title = {{Missing Mergers: On the Unexpected Delay-Time Distribution of Binary Neutron Star Mergers}},
author = {Ryan J. Foley},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.40782},
eprint = {2610.40782},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.40782},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Missing Mergers: On the Unexpected Delay-Time Distribution of Binary Neutron Star Mergers
AU - Ryan J. Foley
PY - 2026
DA - 2026/10/29
PB - arXiv
DO - 10.48550/arxiv.2610.40782
UR - https://arxiv.org/abs/2610.40782
AB - The LIGO-Virgo-KAGRA (LVK) collaboration has detected two binary neutron star (BNS) mergers, GW170817 and GW190425. While no clear electromagnetic counterpart was discovered for GW190425, a coincident counterpart to GW170817 (GRB170817A, AT 2017gfo) was discovered in an early-type galaxy, NGC 4993. Using broad-band photometry and an integrated optical spectrum, we measure a mass-weighted age of 12.4^+0.5_−0.7 Gyr (90% credible) for NGC 4993, which is an estimate of the delay time between star formation and merger for the GW170817 progenitor. BNS systems are expected to have a power-law delay-time distribution (DTD) with a minimum delay time, t_min, of ~10 Myr and index α≈ −1. This canonical DTD, convolved with the cosmic star-formation history, predicts ~16 BNS mergers at delay times of ~100 Myr for every BNS merger at 10 Gyr; under it, the chance of detecting even one merger as old as GW170817 is ~1.8%. The long delay time of GW170817 therefore implies many "missing" shorter-delay mergers if the canonical DTD holds. A DTD with t_min = 12.6 Gyr and α= −1 is favored over the canonical DTD by a factor of 40-140, and over short gamma-ray burst and Galactic double-NS DTDs by factors of 80-280. These results are in tension with current population synthesis models, the Milky Way double NS population, and the SGRB population. Explaining the LVK population requires either a dramatically different separation distribution than commonly assumed, an additional channel with a long delay time and comparable rate to an isolated-binary formation scenario, or a large population of BNS mergers missed in the LVK analysis. This result has wide-ranging implications for binary-population synthesis, chemical enrichment models, connections between BNS mergers and SGRBs, and future gravitational wave observations.
ER -