Eccentricities of Double Neutron Star Binaries

Abstract

Recent pulsar surveys have increased the number of observed double neutron stars (DNSs) in our galaxy enough so that observable trends in their properties are starting to emerge. In particular, it has been noted that the majority of DNSs have eccentricities less than 0.3, surprisingly low for binaries that survive a supernova explosion, which we believe imparts a significant kick to the neutron star. To investigate this trend, we generate many different theoretical distributions of DNS eccentricities using Monte Carlo population synthesis methods. We determine which eccentricity distributions are most consistent with the observed sample of DNS binaries. In agreement with the work of Chaurasia and Bailes, assuming all DNSs are equally as probable to be discovered as binary pulsars, we find that highly eccentric, coalescing DNSs are less likely to be observed because of their accelerated orbital evolution due to gravitational wave emission and possible early mergers. Based on our results for coalescing DNSs, we also find that models with vanishingly or moderately small kicks (σ 50 km s-1) are inconsistent with the current observed sample of such DNSs. We discuss the implications of our conclusions for DNS merger rate estimates, of interest to ground-based gravitational wave interferometers. We find that, although orbital evolution due to gravitational radiation affects the eccentricity distribution of the observed sample, the associated upwards correction factor to merger rate estimates is rather small (typically 10%-40%).