Resolving the cosmic X-ray background with a next-generation high-energy X-ray observatory

TL;DR

This paper reviews recent advances in understanding the cosmic X-ray background and discusses how a next-generation high-energy X-ray observatory could resolve and characterize the majority of accreting supermassive black holes responsible for it.

Contribution

It highlights the potential of a new high-resolution X-ray mission to address unresolved questions about AGN obscuration and SMBH growth.

Findings

NuSTAR has advanced understanding of high-energy AGN emission.

Remaining challenges include understanding AGN obscuration.

Next-generation observatories could resolve most SMBHs contributing to the CXB.

Abstract

The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, is produced primarily by emission from accreting supermassive black holes (SMBHs). The CXB therefore serves as a constraint on the integrated SMBH growth in the Universe and the accretion physics and obscuration in active galactic nuclei (AGNs). This paper gives an overview of recent progress in understanding the high-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB, with an emphasis on results from NASA's NuSTAR hard X-ray mission. We then discuss remaining challenges and open questions regarding the nature of AGN obscuration and AGN physics. Finally, we highlight the exciting opportunities for a next-generation, high-resolution hard X-ray mission to achieve the long-standing goal of resolving and characterizing the vast majority of the accreting SMBHs that produce the CXB.