The High Energy X-ray Probe (HEX-P): Probing the physics of the X-ray corona in active galactic nuclei

The hard, non-thermal X-ray emission in active galactic nuclei (AGN) and black hole X-ray binaries (BHXRBs) is traditionally thought to be produced by a hot cloud of electrons referred to as the corona, which resides in close vicinity to the central black hole. This emission is commonly described by a power law with a high-energy cutoff in the range of a few tens to several hundreds of keV and is suggestive of Comptonization by thermal electrons. While several hypotheses have been proposed to explain the origin, geometry, composition, and physics of the corona, we still lack a clear understanding of this fundamental component of accreting compact objects. NuSTAR has been playing an important role improving our knowledge of X-ray coronæ thanks to its unprecedented sensitivity above 10 keV. However, the constraints provided by NuSTAR are limited to bright and nearby sources. The High Energy X-ray Probe (HEX-P) is a probe-class mission concept that will combine high spatial resolution X-ray imaging and broad spectral coverage (0.2–80 keV) with a sensitivity superior to current facilities (including XMM-Newton and NuSTAR) to address key astrophysical problems. In this paper, we present a set of simulations to highlight the major role that HEX-P will play in further advancing our insights of X-ray coronæ, notably in AGN. We demonstrate how HEX-P will be able to measure key coronal properties in unobscured AGN and track the temporal evolution of these coronæ, which will allow us to determine their electron distribution and test the dominant mechanisms in these systems. Furthermore, we show how HEX-P will be able to accurately estimate the coronal properties of obscured AGN in the local Universe, which will help address fundamental questions about AGN unification. In addition, HEX-P will characterize coronæ in a large sample of luminous quasars at cosmological redshifts for the first time, as well as track the formation and evolution of X-ray coronæ in transient systems in real time. We also demonstrate how HEX-P will be able to locate the X-ray corona and estimate its geometry using spectral-timing techniques. We conclude by discussing HEX-P synergies with other observatories. HEX-P will thus be essential to understand the evolution and growth of black holes over a broad range of mass, distance, and luminosity, and will help uncover the role black holes play in shaping the Universe.