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NORAS - Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey Catalog

HEASARC
Archive

Overview

In the construction of an X-ray-selected sample of galaxy clusters for cosmological studies, the authors have assembled a sample of 495 X-ray sources which were found to show extended X-ray emission in the first processing of the ROSAT All-Sky Survey (RASS I), the Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey Catalog. The sample covers the celestial region with declination >=0 degrees and Galactic latitude |b| >= 20 degrees, and comprises sources with a Position Sensitive Proportional Counter (PSPC) count rate >= 0.06 counts/s and a source extent likelihood of L >= 7. In an optical follow-up identification program, the authors found 378 (76%) of these sources to be clusters of galaxies.

It was necessary to reanalyze 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 in its ability to be robust, to be 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, RASS II. The authors present a catalog of the cluster sources with the X-ray properties obtained as well as a list of the previously flagged extended sources that are found to have a non-cluster counterpart. In their paper, they 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, they extended the search for X-ray cluster sources to the RASS II data for the northern sky region between 9 and 14 hours in right ascension. They included the search for X-ray emission from known galaxy clusters as well as a new investigation of extended X-ray sources. In the course of this search, they found X-ray emission from 85 additional 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 ROSAT-ESO Flux-limited X-ray (REFLEX) Cluster Survey results leads to an estimate of the completeness of the NORAS sample of ROSAT All-Sky Survey (RASS) I extended clusters of about 50% at an X-ray flux of FX(0.1-2.4 keV) = 3 x 10-12 ergs 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 that will be cured with the completion of the continuing Northern ROSAT All-Sky (NORAS) Cluster Survey project.


Catalog Bibcode

2000ApJS..129..435B

References

The northern ROSAT all-sky (NORAS) galaxy cluster survey. I.
X-ray properties of clusters detected as extended X-ray sources.
    Bohringer H., Voges W., Huchra J.P., McLean B., Giacconi R., Rosati P.,
    Burg R., Mader J., Schuecker P., Simic D., Komossa S., Reiprich T.H.,
    Retzlaff J., Trumper J.
   <Astrophys. J. Suppl. Ser. 129, 435 (2000)>
   =2000ApJS..129..435B

Provenance

This table was created by the HEASARC in June 2005 based on CDS table J/ApJS/129/435, table1.dat through table9.dat inclusive.

HEASARC Implementation

As noted in the provenance section, this Browse table was created by the merger of 9 separate CDS tables, corresponding to Tables 1 through 9 of the paper, which contain information on 5 distinct subsamples: (1) the main NORAS galaxy cluster sample of 378 RASS I sources (described in Tables 1 and 5), (2) the 99 RASS I extended sources identified as noncluster objects (Table 2), (3) 18 RASS I X-ray sources where the X-ray emission is `obviously originating from an AGN and in one case from a star, but where a cluster or group of galaxies is also visible' in optical images (Tables 3 and 4), (4) 85 additional Abell (ACO89) clusters detected as RASS II X-ray sources in the region of RA from 9 to 14 hours (Tables 6 and 7), and (5) 21 additional non-Abell clusters in the same RA region which were detected as extended X-ray sources in the RASS II database (Tables 8 and 9). The 484 confirmed galaxy cluster X-ray sources (subsamples 1, 4 and 5) have been given the name prefixes of RXC for `RASS X-ray Cluster', while the remaining 117 X-ray sources from subsamples 2 and 3 have been given the standard RX prefix used for ROSAT X-ray sources. The HEASARC has created a parameter called table_sample which is set to 1 for subsample 1, 2 for subsample 2, and so on, so that users can determine from which subsample any given tabulated source comes.

The other changes that the HEASARC has made are (i) we have created absolute error parameters for the X-ray count rates, fluxes and luminosities by multiplying them by the fractional error parameter, called flux_percent_error in this HEASARC table, and (ii) we have corrected three typographical errors in names which were present in the tables as published: RXC J0125.5+0145 was incorrectly listed as RXC J0124.5+0145 in Table 1, while RXC J0108.2+0210 and RXC J0114.9+0024 were incorrectly listed as RXC J0108.1+0210 and RXC J0114.9+0021, respectively, in Table 5.


Parameters

Name
The position-based designation of the X-ray source. The 484 confirmed galaxy cluster X-ray sources (those with values of table_sample of 1, 4 and 5) were given the name prefixes of RXC for `RASS X-ray Cluster' by the authors, while the remaining 117 noncluster X-ray sources from samples 2 and 3 were given the standard RX prefix used for ROSAT X-ray sources.

Alt_Name
An alternative source name for previously catalogued optical counterparts to the X-ray sources, mainly Abell and Zwicky Cluster names, names of NGC and UGC galaxies forming the central dominant galaxies of groups, and previously identified RASS or other X-ray sources.

Name_Note
This parameter is a flag which indicates a code for a cross-reference for the particular object as follows:

         a: See also Bade et al., 1998A&AS..127..145B
         b: See also Ebeling et al., 1998MNRAS.301..881E
         c: Barton et al., 1996AJ....112..871B
         d: Crawford et al., 1995MNRAS.274...75C
         e: Cooray et al., 1998AJ....115.1388C
         f: Struble & Rood, 1991ApJ...374..395S
         g: See also Tucker et al., 1995ApJ...444..532T
         h: Appenzeller et al., 1998ApJS..117..319A
         i: Wegner et al., 1996ApJS..106....1W
  

RA
The Right Ascension of the X-ray source in the selected equinox. This was given in J2000 decimal degrees to a precision of 0.0001 degrees in the original tables as published (and in their CDS forms).

Dec
The Declination of the X-ray source in the selected equinox. This was given in J2000 decimal degrees to a precision of 0.0001 degrees in the original tables as published (and in their CDS forms).

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Redshift
The redshift of the cluster.

Count_Rate
The PSPC count rate, in ct/s.

Count_Rate_Error
The absolute uncertainty in the count rate, in ct/s. (See the HEASARC_Implementation section above).

Obs_Flux
The X-ray flux of the source, assuming a temperature of 5 keV, in erg/cm^2/s.

Flux
The final calculated `corrected' X-ray flux, in erg/cm^2/s, using the best estimate for the temperature of the source and the redshift of the spectrum, but not the addition of the estimated missing flux, i.e., the flux from beyond the outer radius of significant X-ray emission, Rx.

Flux_Error
The absolute uncertainty in the corrected X-ray flux, in erg/cm^2/s. (See the HEASARC_Implementation section above).

Flux_Percent_Error
The fractional uncertainty in the count rate, X-ray flux, and X-ray luminosity, in percent.

Lx
The rest-frame X-ray luminosity of the cluster of galaxies, in the 0.1-2.4 keV energy band, in erg/s.

Lx_Error
The absolute uncertainty in the X-ray luminosity, in erg/s. (See the HEASARC_Implementation section above).

NH
The absorbing column density in the line of sight to the source, in units of H atoms/cm^2.

ID_Flag
This parameter is a flag which provides information, e.g., on the way that the source position was determined, on sources that have been deblended, etc., as follows:

         ?: if the identification as a cluster leaves some residual doubts
         B: sources that have been deblended
         L: sources that have been analyzed in extra large fields
         C: sources that may be contaminated
       For the position we have used the coordinates determined by the
       moment method with a 3' aperture radius. As an exception, in the
       case of 12 cluster sources we have either chosen to redetermine the
       center position by a moment method with a larger aperture of 5' or
       7.5' or determined a center position by hand. In these sources the
       automatic detection has selected a maximum that is significantly
       offset from the global center of symmetry of the clusters.
         b: 5' moment method
         c: 7.5' moment method
         o: determination by eye
  

Ref_Redshift
The reference codes for the redshift, the key to which is given in ftp://cdsarc.u-strasbg.fr/pub/cats/J/ApJS/129/435/refs.dat

Extent_Parameter
The probability result of the Kolmogorov-Smirnov (KS) test for the source to be a point source, in terms of the parameter P_ext. For very low probabilities for the consistency with a point source, the parameter P_ext was limited to a value of 30 for the entry in the table. A source is considered very likely to be extended if this parameter has at least a value of 2 (point source excluded with 99% probability).

Bestfit_Core_Radius
The best-fitting core radius for the King-model fit, in arcminutes. Note that the core radii determined here are only a qualitative measure of the source extent, and therefore they are not recommended as measures of the statistics of the cluster shapes.

Minimal_Core_Radius
The minimal core radius for the King-model fit which is still consistent within 2-sigma error limits, in arcminutes. Note that the core radii determined here are only a qualitative measure of the source extent, and therefore they are not recommended as measures of the statistics of the cluster shapes.

Hardness_Ratio
The RASS X-ray spectral hardness ratio defined as HR = (H-S)/(H+S), where H is the hard-band count rate and S is the soft-band count rate. The expected values for HR for cluster sources are roughly in the range of 0 to 1.

Hardness_Ratio_Error
The Poisson error in the X-ray spectral hardness ratio.

HR_Sigma_Deviation
The deviation of the measured hardness ratio from the expectation value calculated for the given column density and for a temperature of 5 keV, in units of the 1-sigma error of the hardness ratio, The listed values are limited to a maximum numerical value of 10.

Comments
For a small number of sources (those with table_sample = 3, the cases where AGNs (or a star in one case) lie in the line of sight to clusters), this parameter contains comments about the nature of the object.

Angular_Source_Radius
The radius Rx out to which the source count rate has been integrated, in arcminutes.

Linear_Source_Radius
The radius Rx out to which the source count rate has been integrated, in Mpc.

Table_Sample
This Browse table was created by the merger of 9 separate CDS tables, corresponding to Tables 1 through 9 of the paper, which contain information on 5 distinct subsamples: (1) the main NORAS galaxy cluster sample of 378 RASS I sources (described in Tables 1 and 5), (2) the 99 RASS I extended sources identified as noncluster objects (Table 2), (3) 18 X-ray sources where the X-ray emission is `obviously originating from an AGN and in one case from a star, but where a cluster or group of galaxies is also visible' in optical images (Tables 3 and 4), (4) 85 additional Abell (ACO89) clusters detected as RASS II X-ray sources in the region of RA from 9 to 14 hours (Tables 6 and 7), and (5) 21 additional non-Abell clusters in the same RA region which were detected as extended X-ray sources in the RASS II database (Tables 8 and 9). The HEASARC has created this parameter (which is set to 1 for subsample 1, 2 for subsample 2, and so on) so that users can determine from which subsample any specific tabulated source has been taken.


Contact Person

Questions regarding the NORAS database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 12-Jul-2005