The high energy X-ray probe (HEX-P): bringing the cosmic X-ray background into focus

Abstract

Since the discovery of the cosmic X-ray background (CXB), astronomers have strived to understand the accreting supermassive black holes (SMBHs) 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 Ms. Applying established source detection techniques, we show that HEX-P surveys will reach a flux of ∼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 (∼2000), we showcase HEX-P’s ability to resolve more than 80% of the CXB up to 40 keV into individual sources. The expected precision of HEX-P’s resolved background measurement will allow us to distinguish between population synthesis models of SMBH growth. HEX-P will leverage 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 will explore 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.

PEARLS: NuSTAR and XMM-Newton Extragalactic Survey of the JWST North Ecliptic Pole Time-domain Field II

“…HEX-P will be able to resolve more than 80% of the CXB into individual sources up to 40 keV (Civano et al 2024) and better constrain current population-synthesis models. Its broadband coverage (0.2-80 keV) will allow X-ray spectral analysis of both obscured and unobscured sources and thus more accurately constrain the CT fraction down to 10 −15 erg cm −2 s −1 (Boorman et al 2024;Civano et al 2024).…”

“…HEX-P will be able to resolve more than 80% of the CXB into individual sources up to 40 keV (Civano et al 2024) and better constrain current population-synthesis models. Its broadband coverage (0.2-80 keV) will allow X-ray spectral analysis of both obscured and unobscured sources and thus more accurately constrain the CT fraction down to 10 −15 erg cm −2 s −1 (Boorman et al 2024;Civano et al 2024). Photometric redshift of the associated source 89 Spectroscopic classification (Q for quasars, G for galaxies, S for stars, N/A if no measurement); galaxies are defined as objects without broad emission lines and therefore include type XMM-Newton source name (use "TDFXMM JHHMMSS+DDMM.m") 3-4 X-ray coordinates (J2000) of the source 5 0.5-2 keV band DET_ML 6 0.5-2 keV band vignetting-corrected exposure time (in kiloseconds) at the position of the source 7 0.5-2 keV band net counts of the source (90% confidence upper limit if < mlmin 6) 8 0.5-2 keV band net counts 1σ uncertainty (−99 for upper limits) 9 0.5-2 keV band flux (erg cm −2 s −1 ; 90% confidence upper limit if < mlmin 6) 10 0.5-2 keV band flux 1σ error (−99 for upper limits) 11-16 Same as columns (5)-( 10…”