On the Eccentricities and Merger Rates of Double Neutron Star Binaries and the Creation of “Double Supernovae”

Abstract

We demonstrate that a natural consequence of an asymmetric kick imparted to neutron stars at birth is that the majority of double neutron star binaries should possess highly eccentric orbits. This leads to greatly accelerated orbital decay, due to the enormous increase in the emission of gravitational radiation at periastron as originally demonstrated by Peters (1964). A uniform distribution of kick velocities constrained to the orbital plane would result in ∼24% of surviving binaries coalescing at least 10,000 times faster than an unperturbed circular system. Even if the planar kick constraint is lifted, ∼6% of bound systems still coalesce this rapidly. In a non-negligible fraction of cases it may even be possible that the system could coalesce within 10 years of the final supernova, resulting in what we might term a "double supernova". For systems resembling the progenitor of PSR J0737-3039A, this number is as high as ∼9% (in the planar kick model). Whether the kick velocity distribution extends to the range required to achieve this is still unclear. We do know that the observed population of binary pulsars has a deficit of highly eccentric systems at small orbital periods. In contrast, the longperiod systems favour large eccentricities, as expected. We argue that this is because the short-period highly eccentric systems have already coalesced and are thus selected against by pulsar surveys. This effect needs to be taken into account when using the scale-factor method to estimate the coalescence rate of double neutron star binaries. We therefore assert that the coalesence rate of such binaries is underestimated by a factor of several.

Formation of Double Neutron Star Systems

“…Secondly, we must beware of the observational selection effect (Chaurasia & Bailes 2005) wherein higheccentricity and tight-orbit DNS systems will be removed from the observable sample shortly after their birth due to their small merger timescales. From Eq.…”

Binary and Millisecond Pulsars

“…Below this point, the orbital smearing selection effect discussed in Section 3.1.3 will render the binary undetectable by current surveys. More recent work [73] has suggested that a significant population of highly eccentric binary systems could easily evade detection due to their short lifetimes before gravitational wave inspiral. If this selection effect is significant, then the merger rate estimates quoted below could easily be underestimated by a factor of a few.…”

Vlbi Astrometry of PSR J2222-0137: A Pulsar Distance Measured to 0.4% Accuracy

“…For comparison, among known DNS systems the lowest measured eccentricity is around 0.09 (for PSR J0737-3039; Lyne et al 2004), a factor of over 200 greater than PSR J2222-0137. The majority of DNS systems are expected to be born with a high eccentricity (Chaurasia & Bailes 2005), so despite gravitational wave emission leading to circularization over time, such an extremely low eccentricity would be unexpected. A relatively heavy CO white dwarf companion is the alternative explanation, which would make PSR J2222−0137 an "intermediate-mass binary pulsar" (e.g., Camilo et al 2001).…”