A Disc Reflection Model for Ultra-Soft Narrow-Line Seyfert 1 Galaxies

TL;DR

This paper models the X-ray spectra of ultra-soft narrow-line Seyfert 1 galaxies using a reflection-based approach, revealing high disc ionization and significant non-thermal emission, with implications for understanding their extreme accretion states.

Contribution

It introduces a reflection model for ultra-soft NLS1s, highlighting their high ionization and non-thermal emission dominance, advancing the spectral understanding of these extreme AGN.

Findings

High disc ionization in ultra-soft NLS1s

Eddington ratios between 1 and 20

Significant non-thermal emission component

Abstract

We present a detailed analysis of the XMM-Newton observations of five narrow-line Seyfert 1 galaxies (NLS1s). They all show very soft continuum emission in the X-ray band with a photon index of $\Gamma\gtrsim 2.5$. Therefore, they are referred to as `ultra-soft' NLS1s in this paper. By modeling their optical/UV-X-ray spectral energy distribution (SED) with a reflection-based model, we find indications that the disc surface in these ultra-soft NLS1s is in a higher ionisation state than other typical Seyfert 1 AGN. Our best-fit SED models suggest that these five ultra-soft NLS1s have an Eddington ratio of $\lambda_{\rm Edd}=1-20$ assuming available black hole mass measurements. In addition, our models infer that a significant fraction of the disc energy in these ultra-soft NLS1s is radiated away in the form of non-thermal emission instead of the thermal emission from the disc. Due to their extreme properties, X-ray observations of these sources in the iron band are particularly challenging. Future observations, e.g. from Athena, will enable us to have a clearer view of the spectral shape in the iron band and thus distinguish the reflection model from other interpretations of their broadband spectra.