The High Energy X-ray Probe (HEX-P): Bringing the Cosmic X-ray Background into focus

Kavli Affiliate: Claudio Ricci

| First 5 Authors: Francesca Civano, Xiurui Zhao, Peter Boorman, Stefano Marchesi, Tonima Ananna

| Summary:

Since the discovery of the Cosmic X-ray Background, astronomers have strived
to understand the accreting super massive black holes contributing to its peak
in the 10-40 keV band. Existing soft X-ray telescopes could study this
population up to only 10 keV, and, while NuSTAR (focusing on 3-24 keV) made
great progress, it also left significant uncertainties in characterizing the
hard X-ray population, crucial for calibrating current population synthesis
models. This paper presents an in-depth analysis of simulations of two
extragalactic surveys (deep and wide) with the High-Energy X-ray Probe (HEX-P),
each observed for 2 Megaseconds. Applying established source detection
techniques, we show that HEX-P surveys will reach a flux of $sim$10$^{-15}$
erg s$^{-1}$ cm$^{-2}$ in the 10-40 keV band, an order of magnitude fainter
than current NuSTAR surveys. With the large sample of new hard X-ray detected
sources ($sim2000$), we showcase HEX-P’s ability to resolve more than 80% of
the Cosmic X-ray Background up to 40 keV into individual sources. The
uncertainties expected on HEX-P’s resolved background measurement will allow us
to distinguish between population synthesis models of SMBH growth. HEX-P
leverages accurate broadband (0.5-40 keV) spectral analysis and the combination
of soft and hard X-ray colors to provide obscuration constraints even for the
fainter sources, with the overall objective of measuring the Compton-Thick
fraction. With unprecedented sensitivity in the 10-40 keV band, HEX-P explores
the hard X-ray emission from AGN to flux limits never reached before, thus
expanding the parameter space for serendipitous discoveries. Consequently, it
is plausible that new models will be needed to capture the population HEX-P
will unveil.

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