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WGACAT
*Overview
WGACAT is a point source catalogue generated from all the ROSAT PSPC pointing
observations from Feb 1991 to March 1994 available in the public archive at
HEASARC in September 1994. This catalog has been generated by N.E. White
(HEASARC/GSFC), P. Giommi (ESIS/ESA) and L. Angelini (HEASARC/GSFC) and is a
private research effort, not related to the official catalog planned by the
ROSAT project. The total number of sequence processed were 2624 and include
all the US and the German/UK public archived data. As more data becomes
available the catalog will be updated. The catalog currently (Oct 30, 1994)
contains in excess of 50,000 detections, with more than 45,600 individual
sources.
This catalog, referred to as WGACAT, was generated using an optimized sliding
cell detect algorithm in XIMAGE (first developed for the EXOSAT project). The
inner and outer parts of the images were run separately, to maximize the
sensitivity to source detection. This method is very sensitive to finding
point sources, but can also find spurious sources where there is extended
emission. We have run a quality check on the detect output, and removed the
obvious spurious cases, but users should carefully check the reality of each
source.
**Processing
The processing of the event files was done using a C-shell script to run
the XIMAGE X-ray image analysis package. The XIMAGE software is available
in the FTP anonymous on legacy.gsfc.nasa.gov machine, directory
software/xanadu and it is possible to reproduce and check the entire
catalog. The processing of the ROSAT PSPC data was done using the FITS
event files. The US FITS event files were used as they are stored in the
archive, the German were reformatted to the US FITS event format. In both
cases the files were time ordered to allow a time variability search.
The ROSAT PSPC field was divided in two parts : the first includes
the center of the field roughly up to the inner support structure at 19
arcmin radius. The second is the region outside the inner support and runs
from 18 to 55 arc min. The background, point spread function and vignetting
are very different for the inner and outer regions, and separating them
gives a more sensitive source detection. The inner region was rebinned by
a factor of 8, and the outer part by a factor of 20. For all the detections
a hardness and softness ratio was calculated by splitting the PSPC band
into three bands. For each detected source a chi**2 parameter related to
the time variability has been calculated based on a KS test on the photon
distribution.
**Missed_fields
A few fields were not processed or do not appear in the catalog because:
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* The detection algorythm fails in finding an optimum box size to get
enought boxes free of excesses to calculate the background level.
This effects in total 10 images (in+out).
* The detection fails because too many excesses are found (>=10000)
per image. This effects in total 94 images (in+out). This commonly
occurs if there is a very bright source in the field (e.g. a bright
galactic binary source), or bright extended emission.
* There were no sources detected. This occurs in 173 images (in+out).
In most of case, those images were created from sequence with short
exposure time of a few hundred seconds or less.
* The FITS event file was corrupted. This occurs in 3 sequence.
* the event files were too big (>42Mb) (this occured in only a few cases).
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We are planning to process again the missing field (either the in or out)
by using a different approch to account for the failure in the detect
routine.
**Completness
The PSPC ROSAT sequence processed are all of those US and German/UK
available at HEASARC as of September 1994.
**Cross_correlations
A cross-correlation of the WGA catalogue against all the major catalogs has
allowed a preliminary identification of known optical, radio, X-ray and ir
objects. The results of the cross-correlation is included as parameters in the
WGA catalogue. Only a small fraction of the WGA catalogue (about 10 per cent)
found a matches in the major available catalogue.
The catalogs used include:
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* The Guide Star Catalog
* The Veron 91 catalog
* The Einstein EMSS, IPC, HRI and Slew surveys
* The HEAO 1 A2 and A3 catalogs
* The van Paradijs X-ray Binary catalog (XRBCAT)
* The PMN and 20cm North radio catalogs
* The Lyne Radio Pulsar catalog
* the IRAS FSC catalog
* the Abell catalog
* the ngc2000, Huchra CfA Redshift, mcg, rc3, Hewitt and Burbridge QSO,
Veron91 extragalactic catalogs
* the Positions and Proper motions PMN, BD, HD, SAO, Variable Stars,
Hipparcos Input Catalog, Herbig Emission-Line Star Catalog, CNS3 Nearby
Stars (3rd Ed), the Bright Star, and Be Star catalogs
* the Ritter and Downes/Shara Cataclysmic Variable Catalogs
* the MckSion White Dwarf catalog
* The Simbad source list provided to the ROSAT project
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The results of these correlations are given in various parameters for
each record. The GSC, XID, OID and RID parameters indicate if an
identification has been made (= 1 if it has). The redshift,
VMAG, and various other interesting parameters have been written. Derived
parameters such as FXFO (the optical and X-ray flux ratio) is given. The
FX is a simple conversion, and should be used with care since it depends on
column density and source spectrum. The HII column density from Stark
et al. (1984) is given for the northern sources.
**Available_products
The processing produces for each sequence several products:
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* a smoothed intensity image in GIF format with boxes showing each detection
* a GIF format intensity image centered at each X/Y position
* the FITS format image used to make the detection
* a timing image (a new method for visualizing flux variations) in GIF format
* a color image in GIF format
* a spectrum and lightcurve (GIF and FITS format) for each detection
---
These are not all available at the time of writing (94 Oct 30) but will be
within a few days.
The gif images can be accessed from both the command line and WWW versions
of browse. For the former type `xv` and for the latter click on the
hyperlink.
For each sequence two intensity maps are created one for the inner
region (region descriptor CIRCLE(7860,7860,2295)) rebined by a factor of
8 with a size of 656 X 656, and one for the outer region
(region description CIRCLE(7860,7860,6660) - CIRCLE(7860,7860,2145))
rebin by a factor of 20 with a size of 680. The map are named as follow
xpnnnnnnn_in.img or xpnnnnnnn_out.img, where x is either r for US
data and w for german data, nnnnnnn is the sequence number.
The colour image containing `colour` values, calculated summing the PI
value for each photon arrived at position x, y divided by 100 and
normalized by the total count accumulated in that pixel.
The map are stored as GIF format. The GIF image contains on the left
colour image and on the right the intensity image smoothed with
overlayed the box of the detected sources together with a sky grid.
The map are named as follow xpnnnnnnn_in_col.gif or xpnnnnnnn_out_col.gif
The timing images are a new tool to help identify variable sources. The method
consists in comparing the time arrival distribution of the photons collected
in each pixel with the corresponding distribution of the entire image using a
Kolmogorov-Smirnov (KS) test. The result of the KS test is a $\chi^2$ value
(with 2 d.o.f.) which is then used to assign an intensity value to each pixel.
High intensity chi**2 values are assigned to pixels where the distribution of
photon arrival times is not consistent with that of the entire image. The
`time variability image` so constructed visually shows area where strong time
variations occurred.
**FTP_access
ASCII versions of the catalog has been placed in the anonymous ftp area
on legacy.gsfc.nasa.gov under rosat/wgacat/files. Please read the README
file in that directory.
**Quality
It is imperative that users of WGACAT be aware that it is their responsibility
to check the quality of each detection. As a first step we have checked
by eye the quality of each detection and written a flag from 0 to 10,
in the parameter QFLAG. The visual inspection was of a GIF file made for
each field from XIMAGE, with the detected sources overlayed.
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 the thumb nail
GIFs available through BROWSE).
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. These fields
should be double checked by eye to ensure the detection is real. It is
suggested in this case to inspect the corresponding intensity map.
A QFLAG of 4 indicates that many (typically more than half) the detections are
caused by extended emission, but that there are still good sources (tipically
a small fraction over the total number of detection).
A QFLAG of 3 indicates a source that looks perfectly good, is really
bad. The only case found to date results from a local arc in the detector
caused by a temporary HV breakdown.
We have also flagged all fields taken in the first 2 months of the mission
(before the six month survey phase began) where the processing and satellite
were not stable. These observations have been given a QFLAG of 2. Probably
most of the detection found in those field are correct, but it require
either a checking of the housekeeping parameters or that the processing to
create the final event list was correct.
A QFLAG of 1 indicates most if not all of the detections in that field are
spurious, because of a large extended object.
A QFLAG = 0 means the field has not been inspected.
By default BROWSE will load only the GOOD sample, which has QFLAG >= 5. To
see all the entries use from the `WGACAT_GOOD>` prompt the command
`csam total` to load the entire database.
To check the quality of the detection, the available image files and/or
event list can be used. Those are accesseble directly from the WGACAT
and can be extracted by using the command `xp/image`.
Within the xray account the FITS image products can be view either using
SAOIMAGE or XIMAGE. The first can be invoke from the HEASARC prompt after
having extract the product from the WGACAT database. The second can be
invoke either inside browse (ximage/int n, where n correspond to the entry
number of the list display) or from the HEASARC prompt after having extract
the product.
Within the xray account the GIF products (the timing and color images)
can be view using the public domain package xv either inside or outside of
browse. To invoke the program type in both cases `xv`. Under the WWW
interface click on the hyperlink to the GIF products. In both cases
an X display is required.
**Regions
The inner and outer regions of the PSPC were treated separately. In the
inner region about 19 arc min radius the point spread function and
vignetting are relatively constant, and the sensitivity is a maximum.
In the outer region both the psf and vignetting degrade rapidly and the
sensitivity decreases. The parameter REGION indicates if the detection comes
from the inner (region = i) or outer (region = o).
**Duplicate_Detections
The same source may appear more than once. This can arise because:
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* the field was observed more than once. If a source was observed in the
separate occasion in the inner region and in the outer areas,
then its designation (see ROSAT NEWS No. 32) may be slightly different
because of the lower positional accuracy in the outer region. Also
nearby double sources may be blended in the outer region.
It is adviced, when possible, to always use for the X-ray position
determination the detection nearest the center (the OFFSET parameter
gives the off-axis position in arc min).
* There is a small overlap between the two regions (to ensure sources
dont fall down the gap!) and there may a small number of duplicate
detections found within the same observation one corresponding to a
source detected in the inner region the other detected in the outer region.
Those can be recognized using the parameter ID. These will be removed
in a later version of the catalog.
---
**Count_rate
The count rate has been corrected for vignetting and PSF. Those correction
are energy dependent and the vignetting at 1 keV has been used.
The count/sec are obtained using a constant value for the exposure across
the field. This may underestimate the source count rate, if the source
lies close to the inner support structure and sources detected at the border
of the detector.
**Sensitivity
The PROBABILITY and SNR parameters record, respectively, the probablity
that the detection is due to chance and the signal to noise of the detection.
These were set to be less than 1E-04 and greater than 2, respectively. These
thresholds can be reset using the Browse sp command.
**Samples_Avalilable
The catalog has been divided into three samples called LOWB, HIGHBN and
HIGHBS based on galactic latitude. These are for -2020 and B<-20,
respectively. No attempt has been made to remove the LMC or M31. Use
`csam highbn` e.g. to change to the northern high galactic latitude
survey.
**Parameters
***NAME
The source designation within this catalog. To distinguish these sources from
those found in other searches of the ROSAT archive WGACAT sources use the
designation WGA J1223-3751. WGA stands for White, Giommi and Angelini, the
authors of the catalog, followed by the source coordinate given as `hm-/+d`.
***RA
The source RA (displayed at the current equinox year).
***DEC
The source dec (displayed at the current equinox year).
***JRA
The source RA in J2000.
***JDEC
The source dec in J2000.
***X_PIXEL
The source x coordinate in original pixels (value running from 1-15360).
***Y_PIXEL
The source y coordinate in original pixels (value running from 1-15360).
***LII
The source galactic longitude.
***BII
The source galactic latitude.
***COUNT_RATE
The background subtracted source count rate in counts/sec corrected for
the telescope vignetting and point spread function.
***COUNT_RATE_ERROR
The error on count rate.
***FX
The X-ray flux using a constant correction factor of 1.5*e-11 erg/s/cm**2
(0.05-2.5 keV band)
***VIGNET
The telescope vignetting correction (an increasing number that is 1 on axis,
and a higher value when the source was detected off-axis.
***ERROR_RADIUS
The 90% confidence source error radius. Note this value has been set constant
for all the detections at value 20 arcsec. In most cases, especially on axis,
this is probably closer to 10 arc sec.
***HALF_BOX
The optimum source box size given in original pixels. This has been used
from the detection algorythm to calculate the total number of counts found
in the box.
***CTR_RA
The Image center RA. This corresponds rougly to the pointing position.
***CTR_DEC
The Image center Dec. This correponds rougly to the pointing position.
***CTR_OFFSET
The Offset of the detection position in arcmin from the Image Center.
***TARGET_RA
The Target RA. Note that `TARGET_RA` and `TARGET_DEC` are the
coordinated of the primary target in the field which it can be different
from the CTR_RA and CTR_DEC.
***TARGET_DEC
The Target DEC. Note that `TARGET_RA` and `TARGET_DEC` are the
coordinated of the primary target in the field which it can be different
from the CTR_RA and CTR_DEC.
***OFFSET
The Offset of the Target position in arcmin from the Image Center.
***FIELD
Original field name as found inthe event list (FITS keywords OBJECT).
***ROR
The ROR number. This is a numerical value without the suffix `rp` for US or
`wp` German/UK sequencea and ranges from 100000 up to 900000.
***TIME
The start time of the observation in yr.day
***END_TIME
The end time given as yr.day
***NDETECT
Total number of detections in the image. Note that since each sequence has
been processed individually for the inner or outer region, NDETECT corresponds
to the total number of detections either in the inner of outer region.
***ID_NO
The source number in the field, with number increasing outwards from the
center of the image.
***IMAGEFILE
The FITS image file name (E.g. wp200594_out).
***EVENTFILE
The name of the event file (E.g. wp200594)
***ID
A unique source id for each detection. This is composite by
xxrornumber_region_id, where xx is either `rp` for US sequence and `wp`
for German/UK sequence, region is either `o` for source detected in the
outer region or `i` for source detected in the inner region, id is the
source number detected in the field.
***PROBABILITY
The probability that the detection is a background fluctuation. The
threshold was set to 1.0E-04
***SNR
The signal to noise ratio of the source. The threshold was set at 2.
***QFLAG
A quality flag. In general 1-4 is bad, 5-10 is good. 0 means the field
has not been quality checked. For an ultra clean source use QFLAG>=5.
The default sample given in browse uses QFLAG>=5. To get to the
entire table you must change the sample to TOTAL using `csam total`
from the browse command line.
***REGION
The region of the rosat image used (i =in or o=out)
***ZOOM
The rebinning factor of the image.
***BGND
The background given as counts/original-pixel/s.
***EXPOSURE
The exposure time of field given in seconds.
***SOFTNESS_RATIO
The softness ratio from PI channel numbers 0-40/41-86.
***SOFTNESS_RATIO_ERROR
The error on the softness ratio.
***HARDNESS_RATIO
The hardness ratio from PI channel numbers 87-200/41-86.
***HARDNESS_RATIO_ERROR
The error on the hardness ratio.
***TOTAL_COUNTS
The total number of counts detected.
***TOTAL_COUNTS_ERROR
The error on total_counts.
***LOW_BAND
The number of counts/s in PI channels 0-40.
***LOW_BAND_ERROR
The sqrt of low band.
***MID_BAND
The number of counts/s in PI channels 41-86.
***MID_BAND_ERROR
The sqrt of the low band.
***HIGH_BAND
The number of counts/s in PI channels 87-200.
***HIGH_BAND_ERROR
The sqrt of the high band.
***CHI2
The chi2 variability KS test (>100 is really variable). The chi*2 value is
obtained in comparing the time arrival distribution of the photons collected
in each detection box with the corresponding distribution of the entire image
using a Kolmogorov-Smirnov (KS) test. The result of the KS test is a chi2
value (with 2 d.o.f.).
IMPORTANT NOTE: We have zero'd out the CHI2 time variability parameter. This
is our "science return" for producing the catalog. If your interested
using this parameter, it must be on a strictly collaborative basis.
***XID
Has an id with a pre-rosat X-ray source been made ? 1=yes.
***XID_1
The name of the 1st X-ray ID.
***XID_2
The name of the 2nd X-ray ID.
***XID_3
The name of the 3rd X-ray ID.
***XID_4
The name of the 4th X-ray ID.
***IPC_COUNT_RATE
The count rate detected by the Einstein IPC.
***IPC_COUNT_RATE_ERROR
The error on the IPC count rate.
***IPC_PSPC_RATIO=r*4
The ratio of the IPC/PSPC count rates.
***IPC_PSPC_RATIO_ERROR
The error on the Ratio of the IPC/PSPC count rates.
***OID
Has an Optical id been made? 1=yes.
***OID_1
The name of the first optical ID.
***OID_2
The name of the 2nd optical ID.
***OID_3
The name of the 3rd optical ID.
***OID_4
The name of the 4th optical ID.
***IDNED
Is it identified by NED ? 1=yes (not filled yet)
***IDSIMBAD
Is it in SIMBAD ? 1=yes (not filled yet)
***VMAG
The V Mag of any optical ID.
***FXFO
The X-ray over the optical flux ratio.
***AOX
The Optical to X-ray slope
***REDSHIFT
The Redshift of counterpart.
***CLASS
Source Classification. Type class at the browse prompt for more info
***GSC
Is it in the Guide Star Catalog ? 1=yes.
***GSC_SEQUENCE
GSC Sequence number.
***GSC_REGION
GSC Region number.
***QV_MAG
GSC Quasi-V MAG.
***QB_MAG
GSC Quasi-B MAG.
***EXT
GSC Extended fla.g
***RID
Radio id? 1=yes.
***RID_1
Name of 1st radio ID.
***RID_2
Name of 2nd radio ID.
***RID_3
Name of 3rd radio ID.
***FLUX_80_CM
radio flux (mJy) at 80 cm.
***FLUX_20_CM
radio flux (mJy) at 20 cm.
***FLUX_6_CM
radio flux (mJy) at 6 cm.
***SIHI
radio spectral index between 6 and 20cm.
***ARO
radio optical index.
***IRID
Is an IRAS ID ? 1=yes.
***IRNAME
IRAS name.
***FLUX_10_MICRON
10 micron IRAS flux.
***FLUX_25_MICRON
25 micron IRAS flux.
***FLUX_60_MICRON
60 micron IRAS flux.
***FLUX_100_MICRON
100 micron IRAS flux.
***IDRA
1950 RA of ID star
***IDDEC
1950 Dec of ID star
***IDCAT
catalog from which ID coords obtained.
***IDOFFSET
Offset of ID from ROSAT position given in arcmin.
***NH_21CM
21 cm NH thru the galaxy H cm**-2 from Starke et al.
***BV_COLOR
B-V Color.
**Contact_Persons
Any questions about, or problems with, the WGA catalogue should be
addressed to Nick White, Paolo Giommi and Lorella Angelini
at `wgacat@athena.gsfc.nasa.gov`
**References
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* Giommi, P, Angelini, L., White, N.E. 1994, XIMAGE guide version 2.3
available sending a request at HEASARC.
* Giommi, P., White, N.E., Angelini, L., proceedings of ADASS,
Baltimore US, Sept. 1994.
* Stark et al. 1984 private communication
* White, N.E., Giommi, P, Angelini, L., HEAD meeting Nov. 1994
* White, N.E., Giommi, P, Angelini, L., AAS Tucson (AZ) Jan. 1995.
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