PSR J1641+3627F: a low-mass He white dwarf orbiting a possible high-mass neutron star in the globular cluster M13

TL;DR

This study identifies a faint helium-core white dwarf companion to a millisecond pulsar in M13, suggesting the neutron star may be unusually massive, with implications for neutron star mass distribution and binary evolution.

Contribution

First detailed optical identification of the companion to PSR J1641+3627F, providing constraints on neutron star mass and binary system inclination in a globular cluster.

Findings

Companion is a faint He-core white dwarf with specific mass and temperature.

System likely hosts a neutron star more massive than 1.6 solar masses.

Probability of a neutron star exceeding 1.6 Msun is about 70%.

Abstract

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 \sim 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 0.03 Msun, a radius of 0.033^+0.004_-0.005 Rsun and a surface temperature of 11500^+1900_-1300 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 1.6 Msun. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90 deg and 60 deg, are M_NS,max = 3.1 \pm 0.6 Msun and M_NS,med = 2.4 \pm 0.5 Msun, respectively. On the other hand, assuming also an empirical neutron star mass probability distribution, we found that this system could host a neutron star with a mass of 1.5 \pm 0.1 Msun if orbiting with a low-inclination angle around 40 deg.