PSR J1641+3627F: A Low-mass He White Dwarf Orbiting a Possible High-mass Neutron Star in the Globular Cluster M13

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 ($V\approx 24.3$) 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 $0.23\pm 0.03M_{\odot }$, a radius of ${0.033}_{-0.005}^{+0.004}{R}_{\odot }$, 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 $\sim 70\%$ of probability that this system hosts a neutron star more massive than $1.6M_{\odot }$. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90° and 60°, are $M_{NS,max}=3.1\pm 0.6M_{\odot }$ and $M_{NS,med}=2.4\pm 0.5M_{\odot }$, 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 $1.5\pm 0.1M_{\odot }$ if orbiting with a low-inclination angle around 40°.