Editorial: High-Energy Astrophysics Research Enabled by the Probe-Class Mission Concept HEX-P

Daniel Stern^1*Kristin Madsen²Javier A. García²

• ¹NASA Jet Propulsion Laboratory (JPL), La Cañada Flintridge, United States
• ²Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, Maryland, United States

The High Energy X-ray Probe (HEX-P) is a sensitive, broad-band (0.2-80 keV) X-ray mission concept submitted in response to NASA's Astrophysics Probe Explorer (APEX) Announcement of Opportunity in November 2023 (Figure 1). This was the first time NASA competed an Astrophysics mission of this scale, constrained to a cost cap of ≤ $1.0 B, not including launch, managing the Guest Observer (GO) program, or contributions. Notably, this was also the first time NASA competed a GO facility. This Research Topic includes 17 papers that describe the HEX-P mission and its science. Madsen et al. (2024) provides an overview of the HEX-P instrument and mission profile and García et al. (2024) provides an overview of the HEX-P science. In brief, HEX-P entails two coaligned telescopes: the Low Energy Telescope (LET) which covers 0.2-20 keV with an angular resolution of < 5 ′′ (half-power diameter, or HPD), and the High Energy Telescope (HET) which covers 3-80 keV with an angular resolution of < 20 ′′ (at 20 keV, HPD).APEX missions are intended primarily as community observatories. No data exclusive-use periods are allowed and at least 70% of the baseline five-year mission must be allocated through competed GO opportunities. The remaining time is directed by the science team. For HEX-P, the latter science centers on three primary topics: supermassive black holes (SMBHs), resolved high-energy galactic populations, and time domain and multi-messenger (TDAMM) astrophysics. The PI-led science motivated the instrument and mission design, though providing a powerful, flexible GO capability to serve the broad astrophysics community was always at the core of the HEX-P design. Indeed, this was identified as a Major Strength by the NASA review which highlighted the ability of HEX-P to provide "... a substantial program of GO observations that has a strong potential to produce significant scientific advances."The independent peer review evaluated HEX-P as selectable, noting the mission's "... significant science merit and science implementation strength, coupled with a modest level of technical, management, and cost risk." However, HEX-P ultimately was not selected to proceed to a Phase A study. This is clearly not the end of broadband X-ray astrophysics, and the HEX-P team hopes that this Research Topic will serve as an inspiration for future projects that take on the mantel of addressing this science that is only accessible with an X-ray mission covering a wide bandpass.1 2024) discusses how HEX-P will reveal the X-ray corona, the hot, shining plasma that exists just 44 beyond the event horizon and produces most of the X-rays in the Universe. Measuring the temperature 45 of the corona requires high-energy observations and is a key diagnostic of the poorly understood coronal