A Spectroscopic Survey of X-ray Selected AGN in the Northern XMM-XXL
  Field
  This paper presents a survey of X-ray selected active galactic nuclei (AGN)
with optical spectroscopic follow-up in a $\sim 18\, \rm{deg^2}$ area of the
equatorial XMM-XXL north field. A sample of 8445 point-like X-ray sources
detected by XMM-Newton above a limiting flux of $F_{\rm 0.5-10\, keV} >
10^{-15} \rm\,erg\, cm^{-2}\, s^{-1}$ was matched to optical (SDSS) and
infrared (WISE) counterparts. We followed up 3042 sources brighter than
$r=22.5$ mag with the SDSS BOSS spectrograph. The spectra yielded a reliable
redshift measurement for 2578 AGN in the redshift range $z=0.02-5.0$, with
$0.5-2\rm\, keV$ luminosities ranging from $10^{39}-10^{46}\rm\,erg\,s^{-1}$.
This is currently the largest published spectroscopic sample of X-ray selected
AGN in a contiguous area. The BOSS spectra of AGN candidates show a bimodal
distribution of optical line widths allowing a separation between broad- and
narrow-emission line AGN. The former dominate our sample (70 per cent) due to
the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We
classify the narrow emission line objects (22 per cent of full sample) using
standard BPT diagnostics: the majority have line ratios indicating the dominant
source of ionization is the AGN. A small number (8 per cent of full sample)
exhibit the typical narrow line ratios of star-forming galaxies, or only have
absorption lines in their spectra. We term the latter two classes "elusive''
AGN. We also compare X-ray, optical and infrared color AGN selections in this
field. X-ray observations reveal, the largest number of AGN. The overlap
between the selections, which is a strong function of the imaging depth in a
given band, is also remarkably small. We show using spectral stacking that a
large fraction of the X-ray AGN would not be selectable via optical or IR
colours due to host galaxy contamination.
