Inclination Effects and Beaming in Black Hole X‐Ray Binaries

Abstract

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 gamma of the jet is < 1.24 at the 90% confidence level. ( 2) The X-ray binary 4U 1543 - 47 with i similar to 21 degrees 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 gamma < 1.04. (3) None of the known binaries has cos i < 0.25 or i > 75 degrees. 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 similar to 70 degrees - 75 degrees have significantly more complex X-ray light curves than systems with i less than or similar to 65 degrees. This may be the result of variable obscuration and/or variable height above the disk of the radiating gas.