PSR J0210+5845: Ultra-wide binary pulsar with a B6 V main sequence star companion

Abstract

We report on radio timing observations of PSR\,J0210+5845 that reveal large deviations from typical pulsar spin-down behaviour. We interpret these deviations as being due to the binary motion around the $V=13.5$ star 2MASS\,J02105640$+$5845176, which is coincident in terms of its celestial position and distance with the pulsar. Archival observations and new optical spectroscopy have identified this object as a B6\,V star, with a temperature of $T_ eff 14\,000$\,K and a mass of $M_ c = 3.5$ to $3.8$\,M$_ making it the lowest mass for a main sequence star known to be orbiting a non-recycled pulsar. We find that the timing observations constrain the binary orbit to be wide and moderately eccentric, with an orbital period of $P_ b $\,yr and eccentricity of $. We predict that the next periastron passage will occur between 2030 and 2034. Due to the low companion mass, we find that the probability for a system with the properties of PSR\,J0210+5845 and its binary companion to survive the supernova is low. We show that a low velocity and fortuitously directed natal kick is required for the binary to remain bound during the supernova explosion and we argue that an electron-capture supernova is a plausible formation scenario for the pulsar.