- HEASARC Director: Dr. Alan P. Smale
- HEASARC Associate Director: Dr. Roger Brissenden
- NASA Official: Phil Newman
- Web Curator: Meredith Gibb
- Last Modified: 30-Jun-2011
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CHANGBSCAT - Chandra Galactic Bulge Survey X-Ray Point Source Catalog |
HEASARC Archive |
The Galactic Bulge Survey: Outline and X-ray Observations
Jonker P.G., Bassa C. G., Nelemans G., Steeghs D., Torres M.A.P.,
Maccarone T.J., Hynes R.I., Greiss S., Clem J., Dieball A., Mikles V.J.,
Britt C.T., Gossen L., Collazzi A.C., Wijnands R., In't Zand J.J.M.,
Mendez M., Rea N., Kuulkers E., Ratti E.M., van Haaften L.M., Heinke C.,
Ozel F., Groot P.J., Verbunt F.
<Astrophys. J. Suppl., 194, 18 (2011)>
=2011ApJS..194...18J
The authors also reprocessed and reanalyzed the data that were obtained as part of the Bulge Latitude Survey (Grindlay et al. 2005, ApJ, 635, 920; Hong et al. 2009, ApJ, 706, 223; Chandra observation IDs 7160-7162, 7166-7168, 8199-8204, 9562-9564) that falls in the area they targetted in the GBS using exactly the same reduction and analysis as for their own GBS observations. Because the original Bulge Latitude Survey Chandra observations had an exposure time of approximately 15 ks, the authors selected 2 ks segments to allow for a comparison with their GBS observations. They selected data stretches of 2 ks length taking the start time of the observation from the header of the data, plus 100 s as the starting point of the 2 ks stretches. These data were also obtained using the 'very faint' mode.
The 'very faint' mode provides 5 x 5 pixel information per X-ray event. This allows for a better screening of events caused by cosmic rays. In their analysis, the authors selected events only if their energy falls in the 0.3-8 keV range. They used wavdetect to search for X-ray sources in each of the observations using data covering the full 0.3-8, 0.3-2.5, and 2.5-8 keV energy bands separately. They set the sigthresh in wavdetect to 1 x 10-7, which implies that, for a background count rate constant over the ACIS-I CCDs, there would be 0.1 spurious source detection per observation as about 1 x 106 pixels were searched per observation. However, as explained below, some additional selection criteria were applied, lowering the number of spurious sources.
All sources for which Poisson statistics indicated that the probability of obtaining the number of detected source counts by chance, given the expectation for the local background count rate is lower than 1 x 10-6 were retained. This would be equivalent to a >5-sigma source detection in Gaussian statistics. Next, the authors deleted all sources for which wavdetect was not able to provide an estimate of the uncertainty on the Right Ascension or on the Declination, as this often indicates that all counts fell in a single pixel which could well have been due to faint afterglow events caused by cosmic ray hits. In addition, the authors imposed a three count minimum for source detection.
Since the Chandra GBS observations were designed to overlap near the edges, the authors searched for multiple detections of the same source either in one of the energy sub-bands or in the full energy band. They considered sources with positions falling within 3" of each other to be likely multiple detections of the same source. Finally, they inspected the source list and found two spurious sources caused by the bright readout trail and piled-up core of source number 1.
Source_Number
A unique sequential identification number for each source in
this catalog in order of decreasing source counts, e.g., source number 1 has
the largest number of counts.
Name
The source designation registered with the
Dictionary of Nomenclature of Celestial Objects, using the prefix of 'CXOGBS'
for Chandra X-Ray Observatory Galactic Bulge Survey and the J2000 equatorial
coordinates.
RA
The Right Ascension of the X-ray source in the selected equinox. This
was given in J2000 decimal degree coordinates to a precision of 10-8 degrees
in the original reference.
Dec
The Declination of the X-ray source in the selected equinox. This
was given in J2000 decimal degree coordinates to a precision of 10-8 degrees
in the original reference.
LII
The Galactic Longitude of the X-ray source.
BII
The Galactic Latitude of the X-ray source.
RA_Error
The error in the Right Ascension of the X-ray source, in arcsecs,
as provided by wavdetect. It does not take into account the typical Chandra
boresight uncertainty of 0.6 arcsecs (90% confidence).
Dec_Error
The error in the Declination of the X-ray source, in arcsecs,
as provided by wavdetect. It does not take into account the typical Chandra
boresight uncertainty of 0.6 arcsecs (90% confidence).
Counts
The total number of counts detected in the X-ray source, in the
full 0.3-8 keV energy band.
ObsID
The observation identification number of the observation in which
the X-ray source was detected.
Off_Axis
The off-axis angle of the X-ray source from the pointing center,
in arcminutes.
Num_Detections
The number of individual Chandra pointings in which the
X-ray source was detected.
Hardness_Ratio
The hardness ratio of the X-ray source, defined as the ratio
between the count rate in the 2.5-8 keV band minus that in the 0.3-2.5 keV
band to the count rate in the full 0.3-8 keV energy band (after Kim et al.
2004, ApJS, 150, 19). The authors derived the hardness using XSPEC version
12.6 by determining the count rates in the soft and hard bands taking the
response and ancillary response file for each of the sources into account.
This quantity is given only for sources for which more than 20 counts were
detected and which did not suffer from pile-up, i.e., not for the 3 brightest
sources (1, 2 and 3) or any of the sources with 19 or fewer counts.
Hardness_Ratio_Error
The uncertainty in the hardness ratio of the X-ray
source