A UV to X-ray view of soft excess in type 1 AGNs: I. sample selection and spectral profile

TL;DR

This study analyzes the spectral profiles of soft X-ray excess in 59 unobscured type 1 AGNs, revealing a correlation between soft excess shape and strength, and proposing a hybrid model involving ionized reflection and warm corona contributions.

Contribution

It introduces a robust correlation between soft excess profile and strength, and demonstrates that a hybrid model best explains the observed spectral features in AGNs.

Findings

Powerlaw-like soft excess is more common in stronger soft excess cases.

A cut-off powerlaw can characterize both soft excess types with different Ecut values.

Hybrid models combining ionized reflection and warm corona reproduce observed correlations.

Abstract

A core sample of 59 unobscured type 1 AGNs with simultaneous XMM-Newton X-ray and UV observations is compiled from archive to probe the nature of soft X-ray excess (SE). In the first paper of this series, our focus centers on scrutinizing the spectral profile of the soft excess. Of the sources, $\approx$ 71% (42/59) exhibit powerlaw-like (po-like) soft excess, while $\approx$ 29% (17/59) exhibit blackbody-like (bb-like) soft excess. We show a cut-off powerlaw could uniformly characterize both types of soft excesses, with median Ecut of 1.40 keV for po-like and 0.14 keV for bb-like. For the first time, we report a robust and quantitative correlation between the SE profile and SE strength (the ratio of SE luminosity to that of the primary powerlaw continuum in 0.5 - 2.0 keV), indicating that stronger soft excess is more likely to be po-like, or effectively has a higher Ecut. This correlation cannot be explained by ionized disk reflection alone, which produces mostly bb-like soft excess (Ecut $\sim$ 0.1 keV) as revealed by relxilllp simulation. Remarkably, we show with simulations that a toy hybrid scenario, where both ionized disk reflection (relxilllp, with all reflection parameters fixed at default values except for ionization of the disk) and warm corona (compTT, with temperature fixed at 1 keV) contribute to the observed soft excess, can successfully reproduce the observed correlation. This highlights the ubiquitous hybrid nature of the soft X-ray excess in AGNs, and underscores the importance of considering both components while fitting the spectra of soft excess.