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The merging rate of double neutron stars
The merging rate of double neutron stars
Merging double neutron stars (DNS) are thought to be at the origin of short Gamma Ray Bursts (GRB) and of kilonovae explosions. The merging rate of DNS has thus a great impact on many astrophysical issues, including the interpretation of the gravitational waves signal, of the frequency of short GRB events, and of the chemical properties of stars in galaxies. The time evolution of the rate of merging DNS from a single stellar population is proportional to the distribution of the delay times, or the time elapsed between the birth of the progenitor DNS system and its final merging. We derived a theoretical distribution of the delay times for DNS rooted on the characteristics of the clock controlling the evolution of the binary system (Greggio, Simonetti and Matteucci 2021, MNRAS, 500, 1755). We find that such distribution is characterized by an early peak lasting a few tens of Myr, followed by a power law decline. The shape of the distribution is controlled by two parameters which characterize the DNS system at the epoch when the second neutron star is born: the slope of the distribution of the separations and its minimum value (respectively beta and Amin).
Figure 1
In figure 1 the differential (top panels) and the cumulative (bottom panels) distribution of the delay times of merging DNS for different values of these parameters are shown. The flatter the distribution of the separations (e.g. beta =-1) and the greater the value of Amin (e.g. = 1 Ro), the distribution of the delay times results more populated at the long delays, which enhances the rate of sGRBs in old stellar populations, and retards the pollution of the interstellar medium with the products from kilonovae explosions. We also find that in order to account for the local rate of kilonovae events about 1 % of neutron star progenitors should live in binary sytems which end their evolution as merging DNS within a Hubble time.
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