Investigating the distribution of double neutron stars and unconventional component mass

The mass distribution of double neutron stars (DNSs) can help us to understand the evolution of binary systems, which can also contain information about DNSs induced by gravitational wave radiation. Henceforth, for the 25 pairs of DNS systems so far discovered by radio observations, we have only statistically analysed 13 systems with precisely measured masses of two-component NSs, by employing the classical Gaussian mixture models based on the Akaike and Bayesian information criteria. The result infers that the mass distributions for recycled and non-recycled NSs follow a double-Gaussian and single-Gaussian distribution, respectively. In the conventional scenario of DNS formation, the mass of the first formed recycled NS is higher than that of the second-formed companion NS. However, there exist some unconventional cases with a reverse mass ratio. To explore the phenomenon of this 'unconventional component mass' of DNSs, we employed Monte Carlo sampling. We find that the unconventional DNS systems occupy about 27.7 per cent of all DNSs, which could be a useful reference for the future gravitational wave detection by LIGO/Virgo. Furthermore, we discuss a possible explanation for the unconventional DNS mass ratio, which involves the recycling process and its birth rate. This also has an implication for the mass distribution of the DNS progenitors.