The eROSITA Final Equatorial Depth Survey (eFEDS): Complex absorption and soft excesses in hard X-ray–selected active galactic nuclei
Authors: Waddell S. G. H. , Nandra K. , Buchner J. , Wu Q. , Shen Y. , Arcodia R. , Merloni A. , Salvato M. , Dauser T. , Boller Th. , Liu T. , Comparat J. , Wolf J. , Dwelly T. , Ricci C. , Brownstein J. R. , Brusa M. 
Published: June 2023

Context. The soft excess, a surplus of X-ray photons below 2 keV with respect to a power law, is a feature of debated physical origin found in the X-ray spectra of many type-1 active galactic nuclei (AGN). The eROSITA instrument aboard the Spectrum-Roentgen-Gamma (SRG) mission will provide an all-sky census of AGN. Spectral fitting of these sources can help identify the physical origin of the soft excess.
Aims. The eROSITA Final Equatorial Depth Survey (eFEDS) field, designed to mimic the expected average equatorial depth of the all-sky survey, provides the ideal sample to test the power of eROSITA. The primary goal of this work is to test a variety of models for the soft X-ray emission of AGN (thermal emission, non-thermal emission, ionised absorption, or neutral partial covering absorption) to help identify the physical origin of the soft X-ray spectral complexity. Differences between these models are examined in the context of this sample to understand the physical properties.
Methods. We used Bayesian X-ray analysis to fit a sample of 200 AGN from the eFEDS hard X-ray–selected sample with a variety of phenomenological and physically motivated models. Model selection is performed using the Bayes factor to compare the applicability of each model for individual sources as well as for the full sample, and source properties are compared and discussed. Black hole masses and Eddington ratios were estimated from optical spectroscopy.
Results. We find that 29 sources have evidence for a soft excess at a confidence level > 97.5%, all of which are better modelled by an additional soft power-law, as opposed to thermal blackbody emission. Applying more physically motivated soft excess emission models, we find that 23 sources prefer a warm corona model, while only six sources are best fit with relativistic blurred reflection. Sources with a soft excess show a significantly higher Eddington ratio than the remainder of the sample. Of the remainder of the sample, many sources show evidence for complex absorption, with 29 preferring a warm absorber, and 25 a partial covering absorber. Many (18/26) sources that show significant neutral absorption when modelled with an absorbed power law, in fact show evidence that the absorber is ionised, which has important implications on the understanding of obscured AGN. In contrast to the soft excesses, warm absorber sources show significantly lower Eddington ratios than the remainder of the sample. We discuss the implications of these results for the physical processes in the central regions of AGN.
Conclusions. Spectral fitting with Bayesian statistics is ideal for the identification of complex absorption and soft excesses in the X-ray spectra of AGN, and can allow one to distinguish between different physical interpretations. Applying the techniques from this work to the eROSITA all-sky survey will provide a more complete picture of the prevalence and origin of soft excesses and warm absorbers in type-1 AGN in the local Universe.