A new sample of X-ray selected narrow emission-line galaxies. I. The
  nature of optically elusive AGN
  Using the 3XMM catalogue of serendipitous X-ray sources, and the SDSS-DR9
spectroscopic catalogue, we have obtained a new sample of X-ray selected narrow
emission line galaxies. The standard optical diagnostic diagram and selection
by hard X-ray luminosity expose a mismatch between the optically-based and
X-ray-based classifications. The nature of these misclassified elusive AGN can
be understood in terms of their broader X-ray and optical properties and leads
to a division of this sub-sample into two groups. A little more than half are
likely to be narrow-line Seyfert 1s (NLS1s), so misclassified because of the
contribution of the Broad Line Region (BLR) to their optical spectra. The
remainder have some of the properties of Seyfert 2 (Sy2) AGN; their optical
elusiveness can be explained by optical dilution from the host galaxy plus a
star-formation contribution and by their underluminous optical emission due to
low accretion rates. Because some of the Sy2 sources have very low accretion
rates, are unabsorbed, plus the fact that they lack broad optical emission
lines, they are good candidates to be True Sy2 AGN.
