The soft X-ray excess: NLS1s vs. BLS1s

TL;DR

This study compares the soft X-ray excess in NLS1s and BLS1s, finding it more prominent in NLS1s and likely produced by warm Comptonization, with correlations to accretion rate but not black hole mass.

Contribution

It provides a model-independent measurement of soft excess strength in matched samples of NLS1s and BLS1s, revealing key differences and correlations that inform its origin.

Findings

Soft excess detected in both NLS1s and BLS1s.

Stronger soft excess in NLS1s compared to BLS1s.

Soft excess correlates with photon index and accretion rate.

Abstract

The soft X-ray excess - the excess of X-rays below 2 keV with respect to the extrapolation of the hard X-ray spectral continuum model - is a very common feature among type 1 active galactic nuclei (AGN); yet the nature of the soft X-ray excess is still poorly understood and hotly debated. To shed some light on this issue, we have measured in a model-independent way the soft excess strength in a flux-limited sample of broad-line and narrow-line Seyfert 1 galaxies (BLS1s and NLS1s) that are matched in X-ray luminosity but different in terms of the black hole mass and the accretion rate values, with NLS1s being characterized by smaller MBH and larger accretion rate values. Our analysis, in agreement with previous studies carried out with different AGN samples, indicates that: 1) a soft excess is ubiquitously detected in both BLS1s and NLS1s; 2) the strength of the soft excess is significantly larger in the NLS1 sample, compared to the BLS1 sample; 3) combining the two samples, the strength of the soft excess appears to positively correlate with the photon index as well as with the accretion rate, whereas there is no correlation with the black hole mass. Importantly, our work also reveals the lack of an anticorrelation between the soft excess strength and the luminosity of the primary X-ray component, predicted by the absorption and reflection scenarios. Our findings suggest that the soft excess is consistent with being produced by a warm Comptonization component. Larger, more complete samples of NLS1s and BLS1s are needed to confirm these conclusions.