Probing Black Hole Spin Orientations: the inner disk inclination angle in GRO J1655–40

In an X-ray binary, a spin-orbit misalignment is an angular momenta misalignment between the binary orbit and the black hole (BH). Conventional theory predicts that the rotational axis of the X-ray emitting inner disk regions aligns with the BH spin axis, as does the jet axis. Therefore, measuring the orientation of the inner disk (or jet) probes the BH spin orientation. Our goal is to robustly measure the inner disk inclination angle in GRO J1655–40 (hereafter J1655), whose jet and orbital inclinations are known. We reduce X-ray data from five Swift/XRT observations of J1655 in the high/soft state, which are contaminated by photon pile-up and dust-scattering halos. To eliminate the effect of photon pile-up, we develop an analysis pipeline that extracts the appropriate region of the point spread function. By repurposing the disk continuum fitting technique, typically used to measure BH spin, we measure the inner disk inclination angle. Using Markov Chain Monte Carlo analysis with informative priors, we show that the data favor an inner disk inclination angle of 85º. This is evidence for disk-jet alignment and independent verification of the > 15º spin-orbit misalignment in J1655, assuming the inner disk aligns with the BH spin. We plan to investigate the effects of dust scattering halos that are present in all five Swift/XRT spectra.