The mass of the neutron star in SMC X-1
Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK e-mail: [a.k.f.val-baker;a.j.norton]@open.ac.uk
Accepted: 1 July 2005
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 km s-1. Assuming Sk 160 fills its Roche-lobe, the inclination angle of the system is and in this case we obtain upper limits for the mass of the neutron star as and for Sk 160 as . However if we assume that the inclination angle is , then the ratio of the radius of Sk 160 to the radius of its Roche-lobe is , and the lower limits for the masses of the two stars are and . 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.
Key words: binaries: close / stars: neutron / stars: individual: SMC X-1 / stars: individual: Sk 160 / stars: fundamental parameters
© ESO, 2005