Constraints on the Compact Object Mass in the Eclipsing HMXB XMMU J013236.7+303228 in M33

We present optical spectroscopic measurements of the eclipsing High Mass X-ray Binary XMMU J013236.7+303228 in M33. Based on spectra taken at multiple epochs of the 1.73d binary orbital period we determine physical as well as orbital parameters for the donor star. We find the donor to be a B1.5IV sub-giant with effective temperature T=22,000-23,000 K. From the luminosity, temperature and known distance to M33 we derive a radius of R = 8.9 \pm 0.5 R_sun. From the radial--velocity measurements, we determine a velocity semi-amplitude of K_opt = 63 \pm 12 km/sec. Using the physical properties of the B-star determined from the optical spectrum, we estimate the star's mass to be M_opt = 11 \pm 1 M_sun. Based on the X-ray spectrum, the compact companion is likely a neutron star, although no pulsations have yet been detected. Using the spectroscopically derived B-star mass we find the neutron star companion mass to be M_X = 2.0 \pm 0.4 M_sun, consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_sun found for many millisecond pulsars. We attempt to use as an additional constraint that the B star radius inferred from temperature, flux, and distance, should equate the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but from trying to apply the technique to known systems, we find that the masses are consistently overestimated. Attempting to account for that in our uncertainties, we derive M_X = 2.2^{+0.8}_{-0.6} M_sun and M_opt =13 \pm 4 M_sun. We conclude that precise constraints require detailed modeling of the shape of the Roche surface.