Implications of low neutron star merger rates for gamma-ray bursts, r-process production and Galactic double neutron stars

Abstract

The first multimessenger discovery of a binary neutron star (BNS) merger, GW170817, proved that such mergers can source short gamma-ray bursts (SGRBs) and produce r-process elements. The initial merger rate from this single event was found to be broadly consistent with the SGRB rate, the Milky Way (MW) r-process mass, and the Galactic population of double neutron star (DNS) systems that will merge in a Hubble time. However, only one additional BNS merger has been detected since, and the BNS merger rate has been consistently revised downwards with recent gravitational wave (GW) catalog updates. Analyzing GWTC-4, we find a total BNS merger rate of $28$--$300{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ consisting of $53_{-49}^{+176}{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ in GW170817-like $\sim (1.3,1.3)M_{\odot }$ BNSs (90\% credibility). We revisit the consistency of the BNS merger rate with SGRBs, r-process and Galactic DNSs. In all cases, there is an emerging tension with the BNS (and EM-bright neutron star--black hole, NSBH) merger rate. Comparing to a BNS merger rate of $100{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$, the cosmological SGRB rate is a factor of 3.6--18 higher (despite kilonova followup of SGRBs implying a significant fraction of SGRBs are of BNS origin), the r-process rate is a factor of 0.9--4.1 higher (even though we consider only r-process elements above the second peak), and the rate inferred from Galactic DNSs is a factor of 2.3--5.1 higher than the BNS rate. We discuss how various uncertainties in the inferred rates either alleviate or exacerbate this tension, which point to the various physical processes that can be constrained by such rate comparisons.

updated to match version accepted by ApJL

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