Constraints on the compact object mass in the eclipsing high-mass X-ray binary XMMU J013236.7+303228 in M33

Abstract

We present optical spectroscopic measurements of the eclipsing high-mass X-ray binary (HMXB) XMMU J013236.7+303228 in M33. Based on spectra taken at multiple epochs of the 1.73day binary orbital period we determine physical as well as orbital parameters for the donor star. We find the donor to be a B1.5IV subgiant with effective temperature T = 22, 000-23, 000K. From the luminosity, temperature, and known distance to M33 we derive a radius of R = 8.9 ± 0.5 R ⊙. From the radial-velocity measurements, we determine a velocity semi-amplitude of K opt = 63 ± 12kms-1. Using the physical properties of the B star determined from the optical spectrum, we estimate the star's mass to be M opt = 11 ± 1 M ⊙. 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 ± 0.4 M ⊙, consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 M ⊙ 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 to the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but by 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 ⊙ and M opt = 13 ± 4 M ⊙. We conclude that precise constraints require detailed modeling of the shape of the Roche surface. © 2012. The American Astronomical Society. All rights reserved.