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 approximate to 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 +/- 0.03 M-circle dot, a radius of 0.033-(+0.004)(0.005) R-circle dot, 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 similar to 70% of probability that this system hosts a neutron star more massive than 1.6 M-circle dot. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90 degrees and 60 degrees, are M-NS,M-max = 3.1 +/- 0.6 M-circle dot and M-NS,M-med = 2.4 +/- 0.5 M-circle dot, 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 +/- 0.1 M-circle dot if orbiting with a low-inclination angle around 40 degrees.