The High Energy X-ray Probe: Resolved X-ray Populations in Extragalactic Environments

The High Energy X-ray Probe (HEX-P) is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<20 arcsec FWHM) and broad spectral coverage (0.2–80 keV) with a sensitivity superior to current facilities (including XMM-Newton and NuSTAR). HEX-P will enable revolutionary new insights into a variety of important astrophysical problems. We present simulations of HEX-P observations of a variety of extragalactic environments (e.g., normal, starburst, and passive galaxies) to demonstrate the power of the HEX-P observatory. We show that HEX-P will be capable of detecting hard (4–25 keV) X-ray emission from resolved point-source populations within ∼800 galaxies and integrated emission from ∼6000 galaxies out to 100 Mpc. In such galaxies, HEX-P will: (1) provide unique information about X-ray binary populations, including accretor demographics (black hole and neutron stars), distributions of accretion states and state transition cadences; (2) place order-of-magnitude more stringent constraints on inverse Compton emission associated with particle acceleration in starburst environments; and (3) put into clear context the contributions from X-ray emitting populations to both ionizing the surrounding interstellar medium in low-metallicity galaxies and heating the intergalactic medium in the z > 8 Universe.