The mass of the neutron star in SMC X-1

We present new optical spectroscopy of the eclipsing binary pulsar Sk 160/SMC X-1. From the He I absorption lines, taking heating corrections into account, we determine the radial velocity semi-amplitude of Sk 160 to be 21.8 ± 1.8 km s-1. Assuming Sk 160 fills its Roche-lobe, the inclination angle of the system is i=65.3° ± 1.3° and in this case we obtain upper limits for the mass of the neutron star as Mx= 1.21 ± 0.10 M⊙; and for Sk 160 as Mo= 16.6 ± 0.4 M⊙. However if we assume that the inclination angle is i=90°, then the ratio of the radius of Sk 160 to the radius of its Roche-lobe is β = 0.79 ± 0.02, and the lower limits for the masses of the two stars are Mx= 0.91 ± 0.08 M⊙ and Mo= 12.5 ± 0.1 M⊙. We also show that the He II 4686 Å emission line tracks the motion of the neutron star, but with a radial velocity amplitude somewhat less than that of the neutron star itself. We suggest that this emission may arise from a hotspot where material accreting via Roche lobe overflow impacts the outer edge of an accretion disc