Modeling the Jet Kinematics of the Black Hole Microquasar XTE J1550-564: A Constraint on Spin-Orbit Alignment

Abstract: Measurements of black hole spin made using the continuum-fitting method rely on the assumption that the inclination of the black hole's spin axis to our line of sight is the same as the orbital inclination angle of the host binary system. The X=ray and radio jet data available for the microquasar XTE J1550-564 offer a rare opportunity to test this assumption. Following the work of others, we have modeled these data and thereby determined the inclination angle of the jet axis, which is presumed to be aligned with the black hole's spin axis. We find a jet inclination angle of approximately 71 degrees and place an upper limit on the difference between the spin and orbital inclinations of < 12 degrees (90% confidence). Our measurement tests for misalignment along the line of sight while providing no constraint perpendicular to this plane. Our constraint on the misalignment angle supports the prediction that the spinning black hole in XTE J1550-564 has aligned itself with the orbital plane and provides support for the measurement of its spin via the continuum-fitting method. Our conclusions are based on a simple and reasonable model of a pair of symmetric jets propagating into a low density cavity whose western wall is approximately 20% closer to XTE J1550-564 than its eastern wall.