2021ApJ...908L..46R

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 L₁ 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 L₁ 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.