The MPIfR-MeerKAT Galactic Plane Survey II. The eccentric double neutron star system PSRJ1208-5936 and a neutron star merger rate update

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 J1208-5936, a 28.71-ms recycled pulsar in a double neutron star system with an orbital period of P-b = 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 ((omega) over dot = 0.918(1) deg yr(-1)), resulting in a total system mass of M-t = 2.586(5) M-circle star. We also achieved low-significance constraints on the amplitude of the Einstein delay and Shapiro delay, in turn yielding constraints on the pulsar mass (M-p = 1.26(-0.25)(+0.13)M(circle star)), the companion mass (Mc = 1.32(-0.13)(+0.25)M(circle star)), and the inclination angle (i = 57 +/- 12 degrees). 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 R-MW(new)= 25(-9)(+19) Myr(-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 R-local(new) = 293(-103)(+222) Gpc(-3) yr(-1), which is consistent with LIGO and Virgo O3 observations. With this, we also predict the observation of 10(-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 x 10(-3) M-circle star and 0.4 degrees after two decades of timing, and that in at least a decade from now the detection of (P) over dot(b) and the sky proper motion will serve to make an independent constraint of the distance to the system.