  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.
