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.

Automated summary

The study presents a follow-up investigation on a newly discovered pulsar in the Southern Hemisphere. By observing the system, the researchers determined its orbital parameters, mass, and constraints on its distance. The system's high eccentricity suggests a strong supernova kick during its formation. This finding is important for understanding the formation and evolution of neutron star merger events.