2005A&A...441..685V

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±1.8km/s. 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 M_x=1.21±0.10M_☉ and for Sk 160 as M_o=16.6±0.4M_☉. 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 M_x=0.91±0.08M_☉ and M_o=12.5±0.1M_☉. We also show that the He II 4686Å 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.