The High Energy X-ray Probe (HEX-P): Science Overview

To answer NASA's call for a sensitive X-ray observatory in the 2030s, we present the High Energy X-ray Probe (HEX-P) mission concept. HEX-P is designed to provide the required capabilities to explore current scientific questions and make new discoveries with a broadband X-ray observatory that simultaneously measures sources from 0.2 to 80 keV. HEX-P's main scientific goals include: (1) understand the growth of supermassive black holes and how they drive galaxy evolution; (2) explore the lower mass populations of white dwarfs, neutron stars, and stellar-mass black holes in the nearby universe; (3) explain the physics of the mysterious corona, the luminous plasma close to the central engine of accreting compact objects that dominates cosmic X-ray emission; and (4) find the sources of the highest energy particles in the Galaxy. These goals motivate a sensitive, broadband X-ray observatory with imaging, spectroscopic, and timing capabilities, ensuring a versatile platform to serve a broad General Observer (GO) and Guest Investigator (GI) community. In this paper we present an overview of these mission goals, which have been extensively discussed in a collection of more than a dozen papers presented in this Research Topic volume. The proposed investigations will address key questions in all three science themes highlighted by Astro2020, including their associated priority areas. HEX-P will extend the capabilities of the most sensitive low- and high-energy X-ray satellites currently in orbit, and will complement existing and planned high-energy, time-domain, and multi-messenger facilities in the next decade.