Piecing together the X-ray background: bolometric corrections for active galactic nuclei: Bolometric corrections for AGN

Abstract

The X‐ray background can be used to constrain the accretion history of supermassive black holes (SMBHs) in active galactic nuclei (AGN), with the SMBH mass density related to the energy density due to accretion. A knowledge of the hard X‐ray bolometric correction, κ2‐10 keV, is a vital input into these studies, as it allows us to constrain the parameters of the accretion responsible for SMBH growth. Earlier studies assumed a constant bolometric correction for all AGN, and more recent work has suggested accounting for a dependence on AGN luminosity. Until recently, the variations in the disc emission in the ultraviolet (UV) have not been taken into account in this calculation; we show that such variations are important by construction of optical‐to‐X‐ray spectral energy distributions for 54 AGN. In particular, we use Far Ultraviolet Spectroscopic Explorer (FUSE) UV and X‐ray data from the literature to constrain the disc emission as well as possible. We find evidence for very significant spread in the bolometric corrections, with no simple dependence on luminosity being evident. Populations of AGN such as narrow‐line Seyfert 1 nuclei, radio‐loud and X‐ray‐weak AGN may have bolometric corrections which differ systematically from the rest of the AGN population. We identify other sources of uncertainty including intrinsic extinction in the optical–UV, X‐ray and UV variability and uncertainties in SMBH mass estimates. Our results suggest a more well‐defined relationship between the bolometric correction and Eddington ratio in AGN, with a transitional region at an Eddington ratio of ∼0.1, below which the bolometric correction is typically 15–25, and above which it is typically 40–70. We consider the potential‐implied parallels with the low/hard and high/soft states in Galactic black hole (GBH) accretion, and present bolometric corrections for the GBH binary GX 339−4 for comparison. Our findings reinforce previous studies proposing a multistate description of AGN accretion analogous to that for GBH binaries. Future calculations of the SMBH mass density may need to take into account the possible dependence of κ2‐10 keV on the Eddington ratio.