A High Rate of White Dwarf-Neutron Star Mergers & Their Transients

TL;DR

This paper suggests that white dwarf-neutron star mergers are more common than previously thought, with significant implications for understanding various cosmic phenomena and future transient surveys.

Contribution

It estimates the high rate of white dwarf-neutron star mergers in the galaxy and discusses their potential impact on astrophysical events and gravitational wave detection.

Findings

Estimated merger rate of ~5 x 10^-4 per year in the Milky Way.

Merger rate is 3-6 times less than Type Ia supernovae rate.

Merger rate exceeds observed long-duration gamma-ray burst rate by 5000-10000 times.

Abstract

We argue that the recent groundbreaking discovery by Badenes et al. (2009) of a nearby (~50 pc) white dwarf-neutron star (or black hole) binary (SDSS 1257+5428) with a merger timescale ~500 Myr implies that such systems are common; we estimate that there are of order 10^6 in the Galaxy. Although subject to large uncertainties, the nominal derived merger rate is ~5 x 10^-4 per yr in the Milky Way, just ~3-6 and ~20-40 times less than the Type Ia and core-collapse supernova (SN) rates, respectively. This implies that the merger rate is ~0.5-1 x 10^4 per Gpc^3 per yr in the local universe, ~5000-10000 times more than the observed (beaming-uncorrected) long-duration gamma-ray burst (GRB) rate. We estimate the lower limit on the rate in the Galaxy to be >2.5 x 10^-5 per yr at 95% confidence. We briefly discuss the implications of this finding for the census of long- and short-duration GRBs and their progenitors, the frequency of tight binary companions to Type Ib/c SN progenitors, the origin of ultra-high energy cosmic rays (UHECRs), the formation of rapidly rotating neutron stars and ~2-3 M_sun black holes, the census of faint Ia-like SNe, as well as for upcoming and current transient surveys (e.g., LOSS, PTF, LSST), and for high- (LIGO) and low-frequency (LISA) gravitational wave searches.