NGC2808CXO - NGC 2808 Chandra X-Ray Point Source Catalog
Using new Chandra X-ray observations and existing XMM-Newton X-ray and Hubble Space Telescope far-ultraviolet observations, the authors aim to detect and identify the faint X-ray sources belonging to NGC 2808 in order to understand their role in the evolution of globular clusters. The authors classify the X-ray sources associated with the cluster by analysing their colors and variability. Previous observations with XMM-Newton and far-ultraviolet observations with Hubble are re-investigated to help identify the Chandra sources associated with the cluster. The authors compare their results to population synthesis models and observations of other Galactic globular clusters.
NGC 2808 was observed with the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer-Imager (ACIS-I) on 2007 June 19-21 (28 months after the XMM-Newton observation referred to the reference paper) for two distinct exposures of 46 and 11 kiloseconds. The authors detect 113 sources, of which 16 fall inside the half-mass radius of NGC 2808 and are concentrated towards the cluster core.
Combined Chandra, XMM-Newton, and Hubble Space Telescope observations of the Galactic globular cluster NGC 2808. Servillat M., Dieball A., Webb N.A., Knigge C., Cornelisse R., Barret D., Long K.S., Shara M.M., Zurek D.R. <Astron. Astrophys., 490, 641-654 (2008)> =2008A&A...490..641S (SIMBAD/NED BibCode)
A unique identification number for each Chandra X-ray source in this table.
The NGC 2808 Chandra X-ray source designation, as recommended by the CDS Dictionary of Nomenclature of Celestial Objects, using the '[SDW2008]' prefix (Servillat, Dieball, Webb 2008) and the source number.
This flag parameter is set to 'w' to indicate that the detection method was by WAVDETECT, or to 'h' if it was by hand.
The name of the XMM-Newton counterpart X-ray source, as listed in Servillat, Webb and Barret (2008, A&A, 480, 397), using the '[SWB2008] NGC 2808' prefix recommended by the CDS Dictionary of Nomenclature of Celestial Objects and the (XMM-Newton) source number, where C1-C5 are 5 sources found within the cluster half-mass radius, and 1-92 (excluding 4) are 91 sources found elsewhere in the field.
The Right Ascension of the Chandra X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to an accuracy of 0.01 seconds of time in the original table.
THe Declination of the Chandra X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to an accuracy of 0.01 aecseconds in the original table.
The Galactic Longitude of the Chandra X-ray source.
The Galactic Latitude of the Chandra X-ray source.
The 1-sigma Chandra X-ray source positional error, in arcseconds.
The number of counts in the Chandra source, in the 0.5-8.0 keV energy band.
The unabsorbed 0.5-8 keV flux of the Chandra source, in erg/s/cm2.
The RMS uncertainty in the Chandra source flux, in erg/s/cm2.
The unabsorbed X-ray flux in the 0.5-8 keV band of the XMM-Newton counterpart to the Chandra source, in erg/s/cm2.
The RMS uncertainty in the XMM-Newton source flux, in erg/s/cm2.
The Chandra X-ray source hardness ratio HR_C = (H-S)/(H+S), where S and H are the counts in the 0.5 - 2 keV band and the 2 - 8 keV band, respectively.
The RMS uncertainty in the hardness ratio of the Chandra X-ray source.
The XMM-Newton X-ray source hardness ratio HR_X = (H-S)/(H+S), where S and H are the counts in the 0.5 - 2 keV band and the 2 - 8 keV band, respectively.
The RMS uncertainty in the hardness ratio of the XMM-Newton X-ray source.
The authors performed a generic spectral fitting of the sources with a power law model and the absorption of the cluster (assumed to be 0.12 x 1022 cm-2 for all sources) using XSPEC v12.2 through the procedure acis_extract (FIT_SPECTRA stage). The spectral_index parameter contains the best-fit value of the photon index power law.
The Kolmogorov-Smirnov (KS) probability that the Chandra X-ray source was constant during the observation. The KS test was performed on the extracted and unbinned lightcurves in order to detect variable sources. Seven sources were found to have a KS probability of constancy lower than 10-2, of which two are located in the core of NGC 2808 (source numbers 3 and 16), and the authors confirmed the variability of these sources using Poisson statistics.