Can we reproduce the X-ray background spectral shape using local AGN?

TL;DR

This study analyzes local AGN spectra to assess if their combined emission can replicate the X-ray background spectral shape, revealing similarities and differences with existing models and implications for AGN evolution.

Contribution

It provides the first direct measurement of the combined spectrum of local AGN and compares it to the XRB, challenging existing synthesis models and suggesting limited evolution in AGN properties.

Findings

Local AGN spectrum closely resembles the XRB when shifted to z~1.

Compton-thick AGN contribute about 12% to the spectrum.

AGN account for only ~1% of the XRB intensity.

Abstract

The X-ray background (XRB) is due to the aggregate of active galactic nuclei (AGN), which peak in activity at z~1 and is often modeled as the sum of different proportions of unabsorbed, moderately- and heavily-absorbed AGN. We present the summed spectrum of a complete sample of local AGN (the Northern Galactic Cap of the 58-month Swift/BAT catalog, z<0.2) using 0.4-200keV data and directly determine the different proportions of unabsorbed, moderately and heavily-absorbed AGN that make up the summed spectrum. This stacked low redshift AGN spectrum is remarkably similar in shape to the XRB spectrum (when shifted to z~1), but the observed proportions of different absorption populations differ from most XRB synthesis models. AGN with Compton-thick absorption account for only ~12% of the sample, but produce a significant contribution to the overall spectrum. We confirm that Compton reflection is more prominent in moderately-absorbed AGN and that the photon index differs intrinsically between unabsorbed and absorbed AGN. The AGN in our sample account for only ~1% of the XRB intensity. The reproduction of the XRB spectral shape suggests that strong evolution in individual AGN properties is not required between z~0 and 1.