A new mass estimate for Hercules X-1

Abstract

New optical spectroscopy of the 1.7-d eclipsing binary pulsar HZ Her/Her X-1 is presented. The data are of higher quality than those used in previous studies. After making appropriate corrections for the distortion in the velocity curve due to the strong X-ray heating, the masses of the binary components are derived. The neutron star is found to have a mass of 1.5 ± 0.3 M⊙, in contrast to the last radial velocity study which found a value of less than 1.0 M⊙. Rotational velocity measurements, while insufficiently precise to reject the older mass estimate, suggest that the rotational period of HZ Her differs from the synchronous rate by no more than 20 per cent. Using a recently determined figure for the effective temperature of the unheated face, the distance to Her X-1 is estimated at 6.6 ± 0.4 kpc, higher than prior estimates. The systemic velocity is found to be −65 ± 2 km s−1.