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CHANGALCEN - Chandra Galactic Center Point Source Catalog

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Overview

The Chandra Galactic Center Point Source Catalog is a catalog of 2357 point sources detected during 590 ks of Chandra observations of the 17 by 17 arcminutes field around Sgr A*. This field encompasses a physical area of 40 x 40 pc at a distance of 8 kpc. The completeness limit of the sample at the Galactic Center is 10^31 ergs s^-1 (2.0-8.0 keV), while the detection limit is an order of magnitude lower. The 281 sources detected below 1.5 keV are mainly in the foreground of the Galactic Center, while comparisons to the Chandra deep fields at high Galactic latitudes suggest that only about 100 of the observed sources are background AGNs. The surface density of absorbed sources (not detected below 1.5 keV) falls off as the inverse of the angular separation, Theta, from Sgr A*, in agreement with the distribution of stars in infrared surveys. This demonstrates that the X-ray sources trace the general stellar population at the Galactic Center. Point sources brighter than the completeness limit produce 10% of the flux previously attributed to diffuse emission. The log N - log S distribution of the Galactic Center sources is extremely steep (power-law slope Alpha = 1.7). If this distribution extends down to a flux of 10^-17 ergs cm^-2 s^-1 (10^29 ergs s^-1 at 8 kpc, 2.0-8.0 keV) with the same slope, then point sources would account for all of the previously reported diffuse emission. However, there are numerous diffuse, filamentary structures in the field that also contribute to the total flux, so the 2.0-8.0 keV luminosity distribution must flatten between 10^29 and 10^31 ergs s^-1. Many types of stellar systems should be present in the field at the luminosities to which this study are sensitive. However, the spectra of more than half of the Galactic Center sources are very hard and can be described by a power law (E^-Gamma) with photon index Gamma < 1. Such hard spectra have been seen previously only from magnetically accreting white dwarfs (polars and intermediate polars) and wind-accreting neutron stars (pulsars), suggesting that there are large numbers of these systems in this field.

Catalog Bibcode

2003ApJ...589..225M

References

A Deep Chandra Catalog of X-Ray Point Sources toward the Galactic Center

   Muno, M.P., Baganoff, F.K., Bautz, M.W., Brandt, W.N., Broos, P.S.,
   Feigelson, E.D., Garmire, G.P., Morris, M.R., Ricker, G.R., Townsley, L.K.
  <Ap. J. 589, 225 (2003)>
  = 2003ApJ...589..225M

Provenance

This table was created by the HEASARC in February 2006 based on the version of Table 3 from the paper which was obtained from the electronic ApJ website.

Parameters

Name
The recommended source designation for objects in this catalog, based on the IAU-registered 'CXOGC' prefix for 'Chandra X-ray Observatory Galactic Center' and the J2000 X-ray source position.

RA
The Right Ascension of the X-ray source in the selected equinox. This was given in units of degrees in J2000 equatorial coordinates and to a precision of 0.00001 degrees (0.036 arcseconds) in the original table.

Dec
The Declination of the X-ray source in the selected equinox. This was given in units of degrees in J2000 equatorial coordinates and to a precision of 0.00001 degrees (0.036 arcseconds) in the original table.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Offset
The angular offset of the X-ray source the nominal aim point of the combined observations, Sgr A*, in arcminutes.

Exposure
The sum of the live times for all of the observations in which the source was detected, in seconds (s). This quantity is used in the computation of the photon fluxes. Note that the effective exposure factoring in proximity to chip gaps and bad columns is encoded in the mean value of the ARF. This parameter was given with a precision of 0.1 kiloseconds in the original, as-published table.

PSF_Fraction
The fraction of the point-spread function (PSF) ncompassed by the source extraction region. Sources with fractions near 70% are likely to be in confused regions.

PSF_Energy
The energy at which the PSF was estimated for the calculation of the psf_fraction parameter, in keV; this is 1.5 keV for foreground sources and 4.5 keV for sources at or beyond the Galactic Center.

SB_Counts
The total counts extracted from the source region in the soft band (0.5-2.0 keV).

SB_Bck_Counts
The estimated background counts in the source extraction region in the soft band (0.5-2.0 keV).

SB_Bck_To_Src_Area
The ratio of the areas of the background and source regions, weighted by the number of background counts, in the 0.5-2.0 keV band. The scaling factor to convert counts in the background region into counts in the source region is BSc = Sum[Bi]/Sum[Bi*ASi/ABi], where i is the observation number, Bi are the background counts in the background region i, ABi is the area of background region i, and ASi is the area of the source region i. With this definition, the total counts from all of the background regions (Sum[Bi]) can be divided by BSc to yield the total background in the source region for all of the observations, BK = Sum[Bi*ASi/ABi]. The uncertainty on the estimated background can be computed from the formula, {Sqrt( Sum[Bi] + 0.75 ) + 1}/BSc, for the 1-sigma upper limit from Gehrels (1986, ApJ, 303, 336).

SB_Eff_Area_ARF
The mean value of the effective area function (ARF) in the 0.5-2.0 keV band, in square cm.

M1_Counts
The total counts extracted from the source region in the lower-energy medium (M1) band (2.0-3.3 keV).

M1_Bck_Counts
The estimated background counts in the source extraction region in the lower-energy medium (M1) band (2.0-3.3 keV).

M1_Bck_To_Src_Area
The ratio of the areas of the background and source regions, weighted by the number of background counts, in the 2.0-3.3 keV band. The scaling factor to convert counts in the background region into counts in the source region is BSc = Sum[Bi]/Sum[Bi*ASi/ABi], where i is the observation number, Bi are the background counts in the background region i, ABi is the area of background region i, and ASi is the area of the source region i. With this definition, the total counts from all of the background regions (Sum[Bi]) can be divided by BSc to yield the total background in the source region for all of the observations, BK = Sum[Bi*ASi/ABi]. The uncertainty on the estimated background can be computed from the formula, {Sqrt( Sum[Bi] + 0.75 ) + 1}/BSc, for the 1-sigma upper limit from Gehrels (1986, ApJ, 303, 336).

M1_Eff_Area_ARF
The mean value of the effective area function (ARF) in the 2.0-3.3 keV band, in square cm.

M2_Counts
The total counts extracted from the source region in the higher-energy medium (M2) band (3.3-4.7 keV).

M2_Bck_Counts
The estimated background counts in the source extraction region in the higher-energy medium (M2) band (3.3-4.7 keV).

M2_Bck_To_Src_Area
The ratio of the areas of the background and source regions, weighted by the number of background counts, in the 3.3-4.7 keV band. The scaling factor to convert counts in the background region into counts in the source region is BSc = Sum[Bi]/Sum[Bi*ASi/ABi], where i is the observation number, Bi are the background counts in the background region i, ABi is the area of background region i, and ASi is the area of the source region i. With this definition, the total counts from all of the background regions (Sum[Bi]) can be divided by BSc to yield the total background in the source region for all of the observations, BK = Sum[Bi*ASi/ABi]. The uncertainty on the estimated background can be computed from the formula, {Sqrt( Sum[Bi] + 0.75 ) + 1}/BSc, for the 1-sigma upper limit from Gehrels (1986, ApJ, 303, 336).

M2_Eff_Area_ARF
The mean value of the effective area function (ARF) in the 3.3-4.7 keV band, in square cm.

HB_Counts
The total counts extracted from the source region in the hard band (4.7-8.0 keV).

HB_Bck_Counts
The estimated background counts in the source extraction region in the hard band (4.7-8.0 keV).

HB_Bck_To_Src_Area
The ratio of the areas of the background and source regions, weighted by the number of background counts, in the 4.7-8.0 keV band. The scaling factor to convert counts in the background region into counts in the source region is BSc = Sum[Bi]/Sum[Bi*ASi/ABi], where i is the observation number, Bi are the background counts in the background region i, ABi is the area of background region i, and ASi is the area of the source region i. With this definition, the total counts from all of the background regions (Sum[Bi]) can be divided by BSc to yield the total background in the source region for all of the observations, BK = Sum[Bi*ASi/ABi]. The uncertainty on the estimated background can be computed from the formula, {Sqrt( Sum[Bi] + 0.75 ) + 1}/BSc, for the 1-sigma upper limit from Gehrels (1986, ApJ, 303, 336).

HB_Eff_Area_ARF
The mean value of the effective area function (ARF) in the 4.7-8.0 keV band, in square cm.

FB_Counts
The most likely value of the net source counts in the full band (0.5-8.0 keV): note that this quantity can be negative.

FB_Counts_Limit
This parameter is set to '<' if the source was not considered to be significantly detected in the full-band, i.e, the 90% confidence interval for the net counts was consistent with 0.

FB_Counts_Error
For sources that were significantly detected in the full band, this is the 1-sigma uncertainty derived from the uncertainties in the total observed counts and the estimated background counts. For sources that were not significantly detected (fb_counts_limit = '<') in the full band, this is the 90% upper confidence limit to the net counts.

HR_Soft
The soft color of the X-ray source. Colors are defined according to the expression Color = (H-L)/(H+L), where H and L are the net counts in the higher and lower energy bands, respectively. For the soft color, H is in the 2.0-3.3 keV band, and L is in the 0.5-2.0 keV band. Values are set to -9.000 if the 90% confidence interval of both bands contain zero counts.

HR_Soft_Neg_Err
The lower uncertainty in the value of the soft color. This is set to -9.000 if the harder band counts are consistent with zero.

HR_Soft_Pos_Err
The upper uncertainty in the value of the soft color. This is set to -9.000 if the softer band counts are consistent with zero.

HR_Medium
The medium color of the X-ray source. Colors are defined according to the expression Color = (H-L)/(H+L), where H and L are the net counts in the higher and lower energy bands, respectively. For the medium color, H is in the 3.3-4.7 keV band, and L is in the 2.0-3.3 keV band. Values are set to -9.000 if the 90% confidence interval of both bands contain zero counts.

HR_Medium_Neg_Err
The lower uncertainty in the value of the medium color. This is set to -9.000 if the harder band counts are consistent with zero.

HR_Medium_Pos_Err
The upper uncertainty in the value of the medium color. This is set to -9.000 if the softer band counts are consistent with zero.

HR_Hard
The hard color of the X-ray source. Colors are defined according to the expression Color = (H-L)/(H+L), where H and L are the net counts in the higher and lower energy bands, respectively. For the hard color, H is in the 4.7-8.0 keV band, and L is in the 3.3-4.7 keV band. Values are set to -9.000 if the 90% confidence interval of both bands contain zero counts.

HR_Hard_Neg_Err
The lower uncertainty in the value of the hard color. This is set to -9.000 if the harder band counts are consistent with zero.

HR_Hard_Pos_Err
The upper uncertainty in the value of the hard color. This is set to -9.000 if the softer band counts are consistent with zero.

SB_Flux
The most likely value of the photon flux of the source in the soft band (0.5-2.0 keV), in photons cm^-2 s^-1: note that this quantity can be negative.

SB_Flux_Limit
This parameter is set to '<' if the source was not considered to be significantly detected in the soft band, i.e, the 90% confidence interval was consistent with 0.

SB_Flux_Error
For sources that were significantly detected in the soft band, this is the 1-sigma uncertainty. For sources that were not significantly detected (sb_flux_limit = '<'), this is the 90% upper confidence limit to the flux in this band.

M1_Flux
The most likely value of the photon flux of the source in the M1 band (2.0-3.3 keV), in photons cm^-2 s^-1: note that this quantity can be negative.

M1_Flux_Limit
This parameter is set to '<' if the source was not considered to be significantly detected in the M1 band, i.e, the 90% confidence interval was consistent with 0.

M1_Flux_Error
For sources that were significantly detected in the M1 band, this is the 1-sigma uncertainty. For sources that were not significantly detected (m1_flux_limit = '<'), this is the 90% upper confidence limit to the flux in this band.

M2_Flux
The most likely value of the photon flux of the source in the M2 band (3.3-4.7 keV), in photons cm^-2 s^-1: note that this quantity can be negative.

M2_Flux_Limit
This parameter is set to '<' if the source was not considered to be significantly detected in the M2 band, i.e, the 90% confidence interval was consistent with 0.

M2_Flux_Error
For sources that were significantly detected in the M2 band, this is the 1-sigma uncertainty. For sources that were not significantly detected (m2_flux_limit = '<'), this is the 90% upper confidence limit to the flux in this band.

HB_Flux
The most likely value of the photon flux of the source in the M2 band (4.7-8.0 keV), in photons cm^-2 s^-1: note that this quantity can be negative.

HB_Flux_Limit
This parameter is set to '<' if the source was not considered to be significantly detected in the hard band, i.e, the 90% confidence interval was consistent with 0.

HB_Flux_Error
For sources that were significantly detected in the hard band, this is the 1-sigma uncertainty. For sources that were not significantly detected (hb_flux_limit = '<'), this is the 90% upper confidence limit to the flux in this band.


Contact Person

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

Page Author: Browse Software Development Team
Last Modified: 8-Aug-2006