A new delay time distribution for merging neutron stars tested against Galactic and cosmic data

Abstract

The merging of two neutron stars (MNS) is thought to be the source of short gamma-ray bursts (SGRB) and gravitational wave transients, as well as the main production site of r-process elements like Eu. We have derived a new delay time distribution (DTD) for MNS from theoretical considerations and we have tested it against (i) the SGRB redshift distribution and (ii) the Galactic evolution of Eu and Fe, in particular the [Eu/Fe] versus [Fe/H] relation. For comparison, we also tested other DTDs, as proposed in the literature. To address the first item, we have convolved the DTD with the cosmic star formation rate, while for the second we have employed a detailed chemical evolution model of the Milky Way. We have also varied the DTD of Type Ia SNe (the main Fe producers), the contribution to Eu production from core-collapse SNe, as well as explored the effect of a dependence on the metallicity of the occurrence probability of MNS. Our main results can be summarized as follows: (i) The SGRB redshift distribution can be fitted using DTDs for MNS that produce average time-scales of 300-500 Myr; (ii) If the MNS are the sole producers of the Galactic Eu and the occurrence probability of MNS is constant the Eu production time-scale must be on the order of ≲30 Myr; (iii) Allowing for the Eu production in core-collapse SNe or adopting a metallicity-dependent occurrence probability, allow us to reproduce both observational constraints, but many uncertainties are still present in both assumptions.