WGACAT94 (E0): ROSAT WGACAT X-ray Point Source Catalog; Sept 1994 (19950115)
FILE: wgacat94.e0 (49721 sources)
CONTENTS: ROSAT WGACAT X-ray Point Source Catalog of Sept 1994
DATASET: This file comes in HyperSky dataset E-12; version 19950115. Availability on HyperSky CD-ROM: Vol. 2 (1995)
DESCRIPTION: This datafile contains 49721 X-ray sources from the WGACAT catalog, named after its authors N.E. White (HEASARC/GSFC), P. Giommi (ESIS/ESA), and L. Angelini (HEASARC/GSFC). WGACAT is a point source catalog generated from all ROSAT satellite PSPC pointing observations from February 1991 to March 1994, and available in public archives at HEASARC as of September 1994. The WGACAT catalog is a research effort of the authors, not related to the official catalog planned by the ROSAT project.
The HyperSky datafile is based on the smallest of three versions of the catalog made available for electronic retrieval. The small version contains basic source data, including name, position, quality, and count rate. The quality flag, in particular, gives an indication of the reliability of the source detection. For the entire ROSAT database, this flag can range from 0 to 10, where quality 0-4 is considered bad, and 5-10 is considered good. The WGA catalog contains only those sources with quality flags of 5-10, as described below.
DATA: The HyperSky datafile contains the following data from the catalog:
POSITION: Right Ascension and Declination are based on the catalog 2000 positions. Catalog positions were provided to an accuracy of 0.1 second in Right Ascension and 1" in declination. According to the WGACAT documentation, the 90% confidence source error radius has been set constant for all source detections at a value of 20 arc sec. In most cases, especially on axis, it is probably closer to 10 arc sec.
MAG: No magnitudes or equivalents are provided in the WGA catalog. A set of artificial magnitudes is provided, however, in the HyperSky catalog strictly to enable use of HyperSky's limiting-magnitude controls to permit selective display of WGACAT sources above a desired signal to-noise ratio (SNR) threshold. The artificial magnitude system is based on the relation (from Pogson) used to define visual magnitudes:
m1 - m2 = 2.5 * log10(S1/S2)
where { m1, m2 } are magnitudes, and { S1, S2 } are source strengths. For the HyperSky catalog, the "strongest" source (S1 = SNR 421) is arbitrarily assigned the magnitude m1 = 1.0. Given this, any other source S2 can have its relative magnitude, m2, derived using the relation:
m2 = m1 + 2.5 * log10 (S1/S2);
In this system, the "weakest" WGACAT source (SNR 2.04) has a magnitude of 6.8. The following table summarizes the relation between magnitude and source strength for catalog extremes and intermediate magnitudes:
Magnitude Signal-to-Noise Ratio (SNR)
----------|----------------------------------
1.0 421 (highest WGACAT SNR)
2.0 172
3.0 66
4.0 26
5.0 10.5
6.0 4.2
6.8 2.04 (lowest WGACAT SNR)
With the above table, you can set HyperSky's limiting magnitude (for emission sources) to mask out WGACAT sources below a desired signal-to-noise ratio. Except for this use, the derived source magnitudes have NO significance. It must be emphasized again that these magnitudes are NOT provided in the WGA catalog; only the signal-to-noise ratios are fundamental WGACAT values.
As a note, the WGACAT source having the highest SNR ratio (421) is source J2337+4627. This source is one of about 50 WGACAT entries coincident with LAMBDA And, which (according to Burnham) is a peculiar spectroscopic binary with a 20-day period and bright calcium lines in its spectrum. The other 50-odd WGACAT sources coincident with this star have SNRs ranging from roughly 3 to 58 (or "magnitudes" 6 to 3).
IDENTIFICATION: The derived source name, constructed from the equinox 2000 RA, Dec HHMM+DDMM with a 'J' prepended. This identification should be preceded by the letters WGA to distinguish WGACAT sources from other catalogs also based on ROSAT data. Thus, the high-SNR source associated with LAMBDA And (discussed above) would be fully identified as:
WGA J2337+4627
It should be noted that these WGA catalog names are not unique. Where sources are clustered close together, more than one source will fit within the position accuracy given by the name. In such situations, a unique identifier is used by the WGA catalog itself to distinguish entries. Because of the length of this identifier, it is omitted in the HyperSky file. However, such sources may still be distinguished and related back to the original WGACAT data using their exact equinox 2000 position.
COMMENTS: The comment line contains the following information, provided if available for each radio source:
* The signal-to-noise ratio, preceded by "snr=", given to an accuracy of 2 decimal places.
* The source quality flag, having a value of 5-10 as mentioned above. Explanations of each quality flag (QFLAG) follow, as taken from the WGACAT documentation:
A QFLAG of 9 or 10 indicates a secure detection of a point source, which has been confirmed by visual inspection of each source (using available thumb nail GIFs).
A QFLAG of 8 indicates that the entire field has been inspected, and there was no obvious problem with the detections in the field as a whole, and that it is reasonably secure. This is different from QFLAG 9 or 10 because each source has not been individually identified.
A QFLAG of 5-7 indicates that there is something suspicious in the field. This might simply mean that there is a bright source, which makes the contrast on the image such that its hard to see the fainter sources. But it may also indicate a cluster of detections suggesting extended emission.