X-ray reflection spectroscopy of the black hole binary XTE J1550-564: how inclined is the disk?

Black hole binaries (BHBs) follow distinct patterns during their outbursts, exhibiting two primary X-ray spectral states, hard and soft. Since we observe many outbursts of multiple detected BHBs (their duty cycles range from years to decades), we can trace the evolution of theiraccretion flows in realtime. However, despite the wealth of broadband observations, several important questions prevail, such as how fast these black holes spin, the geometry and the level of disk truncation as they evolve through accretion states. X-ray reflection spectroscopic modelinghas made some strides in recent years in answering these questions. The characteristic relativistically broadened Iron K alpha line and Compton reflection hump, signatures of X-rays reflected off the optically-thick accretion disk, depend on the black hole spin, the inner radius, and the disk inclination, as well as other key properties of the accretion flow. We present detailed reflection spectroscopy of archival X-ray observations of XTE J1550-564 conducted with the Rossi X-ray Timing Explorer (RXTE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA). XTE J1550-564 is an ideal candidate to benchmark our models, since its dynamical characteristics are well-measured.Using the RELXILL model, we show that fits to the reflection spectrum of XTE J1550-564 during its hard and intermediate states lead to inclination estimates of ~30—40 deg, which are significantly lower than both the orbital inclination (~75 deg) and the inclination derived from the radio jet (within 12 deg of the orbital). This large and unexpected disparity is puzzling, and suggests we still do not fully understand the geometry of the reflector and/or the properties of the disk illumination. We discuss these contrasting inclination estimates in the framework of our ongoing efforts to fully characterize the outbursts of XTE J1550-564 using a total of 517 available archival RXTE spectra.