2020ApJ...905...63C

We report on the discovery of the companion star to the millisecond pulsar J1631+3627F in the globular cluster M13. By means of a combination of optical and near-UV high-resolution observations obtained with the Hubble Space Telescope, we identified the counterpart at the radio source position. Its location in the color-magnitude diagrams reveals that the companion star is a faint (V~24.3) He-core white dwarf. We compared the observed companion magnitudes with those predicted by state-of-the-art binary evolution models and found out that it has a mass of 0.23±0.03M_☉, a radius of 0.033_–0.005^+0.004R_☉, and a surface temperature of 11,500_–1300⁺¹⁹⁰⁰ K. Combining the companion mass with the pulsar mass function is not enough to determine the orbital inclination and the neutron star mass; however, the last two quantities become correlated: we found that either the system is observed at a low-inclination angle, or the neutron star is massive. In fact, assuming that binaries are randomly aligned with respect to the observer line of sight, there is a ∼70 % of probability that this system hosts a neutron star more massive than 1.6M_☉. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90° and 60°, are M_NS,{max}=3.1±0.6M_☉ and M_NS,med=2.4±0.5M_☉, respectively. On the other hand, also assuming an empirical neutron star mass probability distribution, we found that this system could host a neutron star with a mass of 1.5±0.1M_☉ if orbiting with a low-inclination angle around 40°.