2005ApJ...623.1017N

We investigate the dependence of observational properties of black hole X-ray binaries on the inclination angle i of their orbits. We find the following: (1) Transient black hole binaries show no trend in their quiescent X-ray luminosities as a function of i, suggesting that the radiation is not significantly beamed. This is consistent with emission from an accretion disk. If the X-rays are from a jet, then the Lorentz factor γ of the jet is <1.24 at the 90% confidence level. (2) The X-ray binary 4U 1543-47 with i∼21° has a surprisingly strong fluorescent iron line in the high soft state. Quantifying an earlier argument by Park et al., we conclude that if the continuum X-ray emission in this source is from a jet, then γ<1.04. (3) None of the known binaries has cosi<0.25 or i>75°. This fact, plus the lack of eclipses among the 20 black hole binaries in our sample, strongly suggests at the 99.5% confidence level that systems with large inclination angles are hidden from view. The obscuration could be the result of disk flaring, as suggested by Milgrom for neutron star X-ray binaries. (4) Transient black hole binaries with i∼70°-75° have significantly more complex X-ray light curves than systems with i≲65°. This may be the result of variable obscuration and/or variable height above the disk of the radiating gas.