On the delay times of merging double neutron stars

TL;DR

This paper derives a theoretical distribution of delay times for merging double neutron stars, exploring how key parameters influence the rate and comparing predictions with astrophysical observations like short GRBs and stellar chemical abundances.

Contribution

It introduces a parametric model for the delay time distribution based on DNS separation characteristics and assesses observational constraints on this model.

Findings

The local DNS merger rate implies about 1% of neutron star progenitors form merging binaries within a Hubble time.

Current observations of short GRBs do not strongly constrain the DDT shape, but favor a shallow distribution.

Chemical abundances in Milky Way stars suggest additional Europium sources beyond DNS mergers, affecting DDT constraints.

Abstract

The merging rate of double neutron stars (DNS) has a great impact on many astrophysical issues, including the interpretation of gravitational waves signals, of the short Gamma Ray Bursts (GRBs), and of the chemical properties of stars in galaxies. Such rate depends on the distribution of the delay times (DDT) of the merging events. In this paper we derive a theoretical DDT of merging DNS following from the characteristics of the clock controlling their evolution. We show that the shape of the DDT is governed by a few key parameters, primarily the lower limit and the slope of the distribution of the separation of the DNS systems at birth. With a parametric approach we investigate on the observational constraints on the DDT from the cosmic rate of short GRBs and the Europium to Iron ratio in Milky Way stars, taken as tracer of the products of the explosion. We find that the local rate of DNS merging requires that about 1 percent of neutron stars progenitors live in binary systems which end their evolution as merging DNS within a Hubble time. The redshift distribution of short GRBs does not yet provide a strong constraint on the shape of the DDT, although the best fitting models have a shallow DDT. The chemical pattern in Milky Way stars requires an additional source of Europium besides the products from merging DNS, which weakens the related requirement on the DDT. At present both constraints can be matched with the same DDT for merging DNS.