Inclination Effects and Beaming in Black Hole X-ray Binaries

We investigate the dependence of observational properties of black hole Xray 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 4U1543–47 with i ∼ 21◦ has a surprisingly strong fluorescent iron line in the high soft state. Quantifying an earlier argument by Park et al. (2004), 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 cos i < 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 (1978) 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. Subject headings: accretion, accretion disks — black hole physics — radiation mechanisms, nonthermal — X-rays: binaries