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 region 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, whose jet and orbital inclinations are known. We reduce X-ray data from five Swift/XRT observations of GRO J1655-40 in the high/soft state, which are contaminated by photon pile-up and dust-scattering halos. To eliminate these effects, we develop an analysis pipeline that extracts the appropriate region of the point spread function and models the dust-scattered emission. By repurposing the disk continuum fitting technique, typically used to measure BH spin, we measure the inner disk inclination angle. This allows us to test the expectation of disk-jet alignment and independently verify the spin-orbit misalignment in GRO J1655–40. Our work offers an important step toward understanding the 3D orientations of spinning BHs.