# The extended Northern ROSAT Galaxy Cluster Survey (NORAS II) I. Survey Construction and First Results

## TL;DR

The paper describes the construction and initial results of the NORAS II galaxy cluster survey based on ROSAT X-ray data, expanding the sample size and enabling combined analysis with the Southern REFLEX II survey for cosmological studies.

## Contribution

It presents the methodology for building the NORAS II survey, including source identification, parameter determination, and survey statistics, extending the previous NORAS survey to a larger, more complete sample.

## Key findings

- NORAS II contains 860 clusters with median redshift 0.102.
- The survey's logN-logS and luminosity functions are consistent with REFLEX II.
- Cosmological parameters derived from NORAS II agree with previous results.

## Abstract

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.