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

Bret D. Lehmer, Kristen Garofali, Breanna A. Binder, Francesca Fornasini, Neven Vulic, Andreas Zezas, Ann Hornschemeier, Margaret Lazzarini, Hannah Moon, Toni Venters, Daniel Wik, Mihoko Yukita, Matteo Bachetti, Javier A. García, Brian Grefenstette, Kristin Madsen, Kaya Mori, Daniel Stern

Published 2023-11-08Version 1

We construct simulated galaxy data sets based on the High Energy X-ray Probe (HEX-P) mission concept to demonstrate the significant advances in galaxy science that will be yielded by the HEX-P observatory. The combination of high spatial resolution imaging ($<$20 arcsec FWHM), broad spectral coverage (0.2-80 keV), and sensitivity superior to current facilities (e.g., XMM-Newton and NuSTAR) will enable HEX-P to detect hard (4-25 keV) X-ray emission from resolved point-source populations within $\sim$800 galaxies and integrated emission from $\sim$6000 galaxies out to 100 Mpc. These galaxies cover wide ranges of galaxy types (e.g., normal, starburst, and passive galaxies) and properties (e.g., metallicities and star-formation histories). 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.