The ROSAT Raster Survey in the North-Ecliptic Pole Field: X-ray
  Catalogue and Optical Identifications
  The North-Ecliptic Pole is an important region for extragalactic surveys.
Deep/wide contiguous surveys are being performed by several space
observatories. We analyse all ROSAT pointed and survey observations within 40
deg2 around the NEP, restricting the field-of-view to the inner 30' radius. We
obtain an X-ray catalogue of 805 sources with 0.5-2 keV fluxes >2.9E-15 cgs, a
factor of three deeper than the ROSAT All-Sky Survey in this field. The
sensitivity and angular resolution of our data are comparable to the eROSITA
All-Sky Survey expectations. We use HEROES optical and near-infrared imaging
photometry from Subaru and CFHT telescopes together with literature catalogues
and a new deep and wide Spitzer survey in the field to identify X-ray sources
and calculate photometric redshifts for the candidate counterparts. In
particular we utilize mid-IR colours to identify AGN X-ray counterparts.
Despite relatively large error circles and faint counterparts, confusion and
systematic errors, we obtain a rather reliable catalogue of 766 optical
counterparts, redshifts and optical classifications. We find a new population
of luminous absorbed X-ray AGN at large redshifts, not recognized in previous
X-ray surveys, but identified in our work due to the unique combination of
survey solid angle, X-ray sensitivity and multiwavelength photometry. We also
use the WISE and Spitzer photometry to identify a sample of 185 AGN selected
purely through mid-IR colours, most of which are not detected by ROSAT. Their
redshifts and upper limits to X-ray luminosity and X-ray to optical flux ratios
are even higher than for the new class of X-ray selected luminous AGN2. This
unique dataset is important as reference for future deep surveys in the NEP
region. Most of the absorbed distant AGN should be readily picked up by
eROSITA, but they require sensitive mid-IR imaging to be recognized as optical
counterparts.
