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Spin parameters of stellar-mass black holes in X-ray binaries are currentlybeing estimated by fitting the X-ray continuum spectra of their accretion diskemission. For this method, it is necessary to know the inclination of theX-ray-producing inner region of the disk. Since the inner disk is expected tobe oriented perpendicular to the spin axis of the hole, the usual practice isto assume that the black hole spin is aligned with the orbital angular momentumvector of the binary, and to estimate the inclination of the latter fromellipsoidal modulations in the light curve of the secondary star. We show thatthe inclination of the disk can be inferred directly if we have both spectraland polarization information on the disk radiation. The predicted degree ofpolarization varies from 0% to 5% as the disk inclination changes from face-onto edge-on. With current X-ray polarimetric techniques the polarization degreeof a typical bright X-ray binary could be measured to an accuracy of 0.1% byobserving the source for about 10 days. Such a measurement would constrain thedisk inclination to within a degree or two and would significantly improve thereliability of black hole spin estimates. In addition, it would provide newinformation on the tilt between the black hole spin axis and the orbitalrotation axis of the binary, which would constrain any velocity kicksexperienced by stellar-mass black holes during their formation.

Inferring the Inclination of a Black Hole Accretion Disk from Observations of its Polarized Continuum Radiation