Detectability of low energy X-ray spectral components in type 1 AGN

TL;DR

This study assesses the detectability of soft excess and cold absorption in type 1 AGN X-ray spectra, estimating that soft excesses are common in these AGN and cold absorption is present in a minority.

Contribution

It provides a correction for observed detection rates, revealing that soft excesses are likely ubiquitous in type 1 AGN spectra, and demonstrates the recovery of spectral features through joint spectral fitting.

Findings

Estimated true percentage of AGN with soft excess: 75±23%.

Cold absorption found in at least 5%, possibly up to 10%, of type 1 AGN.

Soft excess and cold absorption features are reliably recovered with joint spectral fits.

Abstract

In this paper we examine the percentage of type 1 AGN which require the inclusion of a soft excess component and/or significant cold absorption in the modelling of their X-ray spectra obtained by XMM-Newton. We do this by simulating spectra which mimic typical spectral shapes in order to find the maximum detectability expected at different count levels. We then apply a correction to the observed percentages found for the Scott et al. (2011) sample of 761 sources. We estimate the true percentage of AGN with a soft excess component to be 75+/-23%, suggesting that soft excesses are ubiquitous in the X-ray spectra of type 1 AGN. By carrying out joint fits on groups of low count spectra in narrow z bins in which additional spectral components were not originally detected, we show that the soft excess feature is recovered with a mean temperature kT and blackbody to power-law normalisation ratio consistent with those of components detected in individual high count spectra. Cold absorption with nH values broadly consistent with those reported in individual spectra are also recovered. We suggest such intrinsic cold absorption is found in a minimum of ~5% of type 1 AGN and may be present in up to ~10%.