  In this work, we test the hypothesis that narrow-line Seyfert 1 galaxies
(NSL1s) are active galactic nuclei in their early phase and are therefore
younger and more active than the more common broad-line Seyfert 1 galaxies
(BLS1s). If that is true, then NLS1s should, on average, have lower black hole
masses and higher accretion rates than BLS1s. To test this, we use a sample of
35 NLS1s and 54 BLS1s with similar X-ray luminosity distributions and good
XMM-Newton observations. To determine the black hole mass, we apply an X-ray
scaling method that is independent of any assumptions on the broad-line region
dynamics and the inclination of the objects. We find that, on average, NLS1s
have lower black hole masses, but the difference between the average black hole
masses of NLS1s and BLS1s in our sample is only marginally significant (at the
2.6 sigma level). According to a Kolmogorov-Smirnov test, the distribution of
black hole mass values of NLS1s is different from that of BLS1s at the 99%
confidence level. Even stronger differences between NLS1s and BLS1s are
inferred when the accretion rate distributions of NLS1s are compared to BLS1s,
suggesting that the two populations are indeed distinct. Our study also
indicates that the black hole mass values (both for NLS1s and BLS1s) determined
with the X-ray scaling method are fully consistent with those obtained using
reverberation mapping.
