The High Energy X-ray Probe (HEX-P): Tracking Black Hole X-ray Binary Outbursts

HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10 arcsec FWHM) and broad spectral coverage (0.1-150 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. Using this next-generation general purpose X-ray observatory to probe relativistic X-ray reflection will place unparalleled constraints on physical properties such as the black hole spin parameter, accretion disk properties including radial truncation, gas density, and ionization profile, as well as the physical properties of the compact corona. We simulate a set of HEX-P observations of a galactic stellar-mass black hole binary outburst similar to the bright 2019 outburst of EXO 1846-031 (Negoro et al. 2019), taken in a pattern similar to the existing NuSTAR observations of the outburst. We illustrate the ability of the data to produce constraints on parameters of interest through relatively short observations, highlighting the clear improvements over measurements made using current instruments. By producing multiple simulated observations of the source, we show how tracking an X-ray binary outburst with the proposed instrument will probe the evolution of the physical properties of the system throughout the outburst, as the source passes through multiple spectral states. This will enable gaining a definitive understanding of accretion disk truncation and of coronal geometry. We discuss how HEX-P will revolutionize relativistic reflection measurements by being able to break degeneracies between parameters and drastically reduce the systematic uncertainties of black hole spin measurements. More information on HEX-P, including the full team list, is available at https://hexp.org.