Probing the masses and radii of donor stars in eclipsing X-ray binaries with the Swift burst alert telescope

Physical parameters of both the mass donor and compact object can be constrained in X-ray binaries with well-defined eclipses, as our survey of wind-fed supergiant X-ray binaries IGR J16393-4643, IGR J16418-4532, IGR J16479-4514, IGR J18027-2016, and XTE J1855-026 reveals. Using the orbital period and Kepler's third law, we express the eclipse half-angle in terms of radius, inclination angle, and the sum of the masses. Pulse-timing and radial velocity curves can give masses of both the donor and compact object as in the case of the "double-lined" binaries IGR J18027-2016 and XTE J1855-026. The eclipse half angles are ${15}_{-2}^{+3}$, ${31.7}_{-0.8}^{+0.7}$, 32 ± 2,34 ± 2, and $33.6\pm 0.7$ degrees for IGR J16393-4643, IGR J16418-4532, IGR J16479-4514, IGR J18027-2016, and XTE 1855-026, respectively. In wind-fed systems, the primary not exceeding the Roche-lobe size provides an upper limit on system parameters. In IGR J16393-4643, spectral types of B0 V or B0-5 III are found to be consistent with the eclipse duration and Roche-lobe, but the previously proposed donor stars in IGR J16418-4532 and IGR J16479-4514 were found to be inconsistent with the Roche-lobe size. Stars with spectral types O7.5 I and earlier are possible. For IGR J18027-2016, the mass and radius of the donor star lie between 18.6–19.4 $M_{\odot }$ and 17.4–19.5 $R_{\odot }$. We constrain the neutron star mass between 1.37 and 1.43 $M_{\odot }$.We find the mass and radius of the donor star in XTE J1855-026 to lie between 19.6–20.2 $M_{\odot }$ and 21.5–23.0 $R_{\odot }$. The neutron star mass was constrained to 1.77–1.82 $M_{\odot }$. Eclipse profiles are asymmetric in IGR J18027-2016 and XTE J1855-026, which we attribute to accretion wakes.