The Northern ROSAT All‐Sky (NORAS) Galaxy Cluster Survey. I. X‐Ray Properties of Clusters Detected as Extended X‐Ray Sources

Abstract

In the construction of an X-ray selected sample of galaxy clusters for cosmological studies, we have assembled a sample of 495 X-ray sources found to show extended X-ray emission in the first processing of the ROSAT All-Sky Survey. The sample covers the celestial region with declination δ ≥ 0 • and galactic latitude |b II | ≥ 20 • and comprises sources with a count rate ≥ 0.06 counts s −1 and a source extent likelihood of 7. In an optical follow-up identification program we find 378 (76%) of these sources to be clusters of galaxies.It was necessary to reanalyse the sources in this sample with a new X-ray source characterization technique to provide more precise values for the X-ray flux and source extent than obtained from the standard processing. This new method, termed growth curve analysis (GCA), has the advantage over previous methods to be robust, easy to model and to integrate into simulations, to provide diagnostic plots for visual inspection, and to make extensive use of the X-ray data. The source parameters obtained assist the source identification and provide more precise X-ray fluxes. This reanalysis is based on data from the more recent second processing of the ROSAT Survey. We present a catalogue of the cluster sources with the X-ray properties obtained as well as a list of the previously flagged extended sources which are found to have a non-cluster counterpart. We discuss the process of source identification from the combination of optical and X-ray data.To investigate the overall completeness of the cluster sample as a function of the X-ray flux limit, we extent the search for X-ray cluster sources to the data of the second processing of the ROSAT Survey for the northern sky region between 9 h and 14 h in right ascension. We include the search for X-ray emission of known clusters as well as a new investigation of extended X-ray sources. In the course of this search we find X-ray emission from additional 85 Abell clusters and 56 very probable cluster candidates among the newly found extended sources. A comparison of the X-ray cluster number counts of the NORAS sample with the REFLEX Cluster Survey results leads to an estimate of the completeness of the NORAS sample of RASS I extended clusters of about 50% at an X-ray flux of F x (0.1 − 2.4keV) = 3 × 10 −12 erg s −1 cm −2 . The estimated completeness achieved by adding the supplementary sample in the study area amounts to about 82% in comparison to REFLEX. The low completeness introduces an uncertainty in the use of the sample for cosmological statistical studies which will be cured with the completion of the continuing Northern ROSAT All-Sky -3 -(NORAS) cluster survey project.

NGC 55: a disc galaxy with flat abundance gradients

“…The k-correction from Böhringer et al (2000) has been applied to obtain the restframe X-ray luminosity. From the rest-frame L 2−10keV we estimated the star formation rate using:…”

A pan-chromatic view of the galaxy cluster XMMU J1230.3+1339 atz= 0.975

“…We applied the growth curve analysis (GCA) method of Böhringer et al (2000) to obtain an accurate 0.5−2 keV flux measurement of the cluster as a function of radius. Since the effective exposure time and the point-spread-function (PSF) at the cluster source are significantly better in field 0112552101, we focus the quantitative analysis on this single observation and use the second field 0106060401 only for a consistency check.…”

The galaxy clusterYSZ−LXandYSZ−Mrelations from the WMAP 5-yr data

“…In this work, we assume symmetric beams, for which the transfer function b is computed, in each frequency channel, from the noise-weighted average of the transfer functions of individual differential assemblies (a similar approach was used in Delabrouille et al 2009). (Pratt et al 2009;Arnaud et al 2010 We construct our cluster sample from the largest published X-ray catalogues: NORAS (Böhringer et al 2000) and REFLEX (Böhringer et al 2004), both constructed from the ROSAT AllSky Survey. We merge the cluster lists given in Tables 1, 6 and 8 from Böhringer et al (2000) and Table 6 from Böhringer et al (2004) and since the luminosities of the NORAS clusters are given in a standard cold dark matter (SCDM) cosmology (h = 0.5, Ω M = 1), we converted them to the WMAP5 cosmology.…”

“…(Pratt et al 2009;Arnaud et al 2010 We construct our cluster sample from the largest published X-ray catalogues: NORAS (Böhringer et al 2000) and REFLEX (Böhringer et al 2004), both constructed from the ROSAT AllSky Survey. We merge the cluster lists given in Tables 1, 6 and 8 from Böhringer et al (2000) and Table 6 from Böhringer et al (2004) and since the luminosities of the NORAS clusters are given in a standard cold dark matter (SCDM) cosmology (h = 0.5, Ω M = 1), we converted them to the WMAP5 cosmology. We also convert the luminosities of REFLEX clusters from the basic ΛCDM cosmology (h = 0.7,…”