PSR J1810+1744: Companion Darkening and a Precise High Neutron Star Mass

Abstract Keck-telescope spectrophotometry of the companion of PSR J1810+1744 shows a flat, but asymmetric light-curve maximum and a deep, narrow minimum. The maximum indicates strong gravity darkening (GD) near the L 1 point, along with a heated pole and surface winds. The minimum indicates a low underlying temperature and substantial limb darkening. The GD is a consequence of extreme pulsar heating and the near-filling of the Roche lobe. Light-curve modeling gives a binary inclination i = 65.°7 ± 0.°4. With the Keck-measured radial-velocity amplitude K c = 462.3 ± 2.2 km s −1 , this gives an accurate neutron star mass M NS = 2.13 ± 0.04 M ⊙ , with important implications for the dense-matter equation of state. A classic direct-heating model, ignoring the L 1 gravitational darkening, would predict an unphysical M NS > 3 M ⊙ . A few other “spider” pulsar binaries have similar large heating and fill factor; thus, they should be checked for such effects.