The MPIfR-MeerKAT Galactic Plane Survey: II. The eccentric double neutron star system PSR J1208a-5936 and a neutron star merger rate update

Abstract

The MPIfR-MeerKAT Galactic Plane survey at L-band (MMGPS-L) is the most sensitive pulsar survey in the Southern Hemisphere, providing 78 discoveries in an area of 900 sq. deg. Here, we present a follow-up study of one of these new discoveries, PSR J1208a-5936, a 28.71-ms recycled pulsar in a double neutron star system with an orbital period of Pb=0.632 days and an eccentricity of e=0.348, merging within the Hubble time. Through timing of almost one year of observations, we detected the relativistic advance of periastron ( Ì = 0.918(1) deg yra-1), resulting in a total system mass of Mt=2.586(5) M·. We also achieved low-significance constraints on the amplitude of the Einstein delay and Shapiro delay, in turn yielding constraints on the pulsar mass (Mp = 1.26a-0.25+0.13 M·), the companion mass (Mc = 1.32a-0.13+0.25 M·), and the inclination angle (i=57 ± 12). This system is highly eccentric compared to other Galactic field double neutron stars with similar periods, possibly hinting at a larger-than-usual supernova kick during the formation of the second-born neutron star. The binary will merge within 7.2(2) Gyr due to the emission of gravitational waves, making it a progenitor of the neutron star merger events seen by ground-based gravitational wave observatories. With the improved sensitivity of the MMGPS-L, we updated the Milky Way neutron star merger rate to be RMWnew = 25a-9+19 Myra-1 within 90% credible intervals, which is lower than previous studies based on known Galactic binaries owing to the lack of further detections despite the highly sensitive nature of the survey. This implies a local cosmic neutron star merger rate of Rlocalnew = 293a-103+222 Gpca-3 yra-1, which is consistent with LIGO and Virgo O3 observations. With this, we also predict the observation of 10a-4+8 neutron star merger events during the LIGO-Virgo-KAGRA O4 run. We predict the uncertainties on the component masses and the inclination angle will be reduced to 5- 10a-3 M· and 0.4 after two decades of timing, and that in at least a decade from now the detection of b and the sky proper motion will serve to make an independent constraint of the distance to the system.