The MPIfR-MeerKAT Galactic Plane Survey. II. The eccentric double neutron star system PSR J1208−5936 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 (ω̇ = 0.918(1) deg yr⁻¹), resulting in a total system mass of M_t = 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 (M_p = 1.26_−0.25^+0.13 M_⊙), the companion mass (M_c = 1.32_−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 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 × 10⁻³ M_⊙ and 0.4° after two decades of timing, and that in at least a decade from now the detection of Ṗ_b and the sky proper motion will serve to make an independent constraint of the distance to the system.