The extended Northern ROSAT Galaxy Cluster Survey (NORAS II) I. Survey
  Construction and First Results
  As the largest, clearly defined building blocks of our Universe, galaxy
clusters are interesting astrophysical laboratories and important probes for
cosmology. X-ray surveys for galaxy clusters provide one of the best ways to
characterise the population of galaxy clusters. We provide a description of the
construction of the NORAS II galaxy cluster survey based on X-ray data from the
northern part of the ROSAT All-Sky Survey. NORAS II extends the NORAS survey
down to a flux limit of 1.8 x 10^(-12) erg s^-1 cm^-2 (0.1 - 2.4 keV)
increasing the sample size by about a factor of two. The NORAS II cluster
survey now reaches the same quality and depth of its counterpart, the Southern
REFLEX II survey, allowing us to combine the two complementary surveys. The
paper provides information on the determination of the cluster X-ray
parameters, the identification process of the X-ray sources, the statistics of
the survey, and the construction of the survey selection function, which we
provide in numerical format. Currently NORAS II contains 860 clusters with a
median redshift of z = 0.102. We provide a number of statistical functions
including the logN-logS and the X-ray luminosity function and compare these to
the results from the complementary REFLEX II survey. Using the NORAS II sample
to constrain the cosmological parameters, sigma_8 and Omega_m, yields results
perfectly consistent with those of REFLEX II. Overall, the results show that
the two hemisphere samples, NORAS II and REFLEX II, can be combined without
problems to an all-sky sample, just excluding the Zone-of-Avoidance.
