2006PASP..118..392D

Accurate measurements of neutron star masses are needed to constrain the equation of state of neutron star matter–of importance to both particle physics and the astrophysics of neutron stars–and to identify the evolutionary track of the progenitor stars that form neutron stars. The best measured values of the mass of 4U 0900-40 (=Vela XR-1), 1.86±0.16 M_☉(Barziv et al.) and 1.93±0.20 M_☉(Abubekerov et al.), make it a leading candidate for the most massive neutron star known. The direct relationship between the maximum mass of neutron stars and the equation of state of ultradense matter makes 4U 0900-40 an important neutron star mass to determine accurately. The confidence intervals on previous mass estimates obtained from observations that include parameters determined by nondynamical methods are not small enough to significantly restrict possible equations of state. We describe here a purely dynamical method for determining the mass of 4U 0900-40, an X-ray pulsar, using the reprocessed UV pulses emitted by its B0.5 Ib companion. One can derive the instantaneous radial velocity of each component through simultaneous X-ray and UV observations at the two quadratures of the system. The Doppler shift caused by the primary's rotational velocity and the illumination pattern of the X-rays on the primary, two of the three principal contributors to the uncertainty on the derived mass of the neutron star, almost exactly cancel out by symmetry in this method. A heuristic measurement of the mass of 4U 0900-40, using observations obtained previously with the High Speed Photometer on the Hubble Space Telescope, is given in an appendix.