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NGC2808XMM - NGC 2808 XMM-Newton X-Ray Point Source Catalog


Galactic globular clusters harbour binary systems that are detected as faint X-ray sources. These close binaries are thought to play an important role in the stability of the clusters by liberating energy and delaying the inevitable core collapse of globular clusters. The inventory of close binaries and their identification is therefore essential.

This table contains some of the results, namely an X-ray source catalog, from XMM-Newton observations of the Galactic globular cluster NGC 2808. Ths authors use X-ray spectral and variability analysis combined with ultra-violet observations made with the XMM-Newton optical monitor and published data from the Hubble Space Telescope to identify sources associated with the clusters. They compare the results of their observations with estimates from population synthesis models.

Five sources out of 96 X-ray sources detected above 4-sigma significance are likely to be related to NGC 2808. The authors find one quiescent neutron star low-mass X-ray binary candidate in the core of NGC 2808, and propose that the majority of the central sources in NGC 2808 are cataclysmic variables. An estimation leads to 20 +\- 10 cataclysmic variables with luminosity above 4.25 x 1031 erg s-1. Millisecond pulsars could also be present in the core of NGC 2808, and some sources outside the half-mass radius could possibly be linked to the cluster.

NGC 2808 was observed on February 1st 2005, for 41.8 kiloseconds (ks) with the three European Photon Imaging Cameras (EPIC MOS1, MOS2 and pn) on board the XMM-Newton observatory, in imaging mode, using a full frame window and a medium filter.

Catalog Bibcode



XMM-Newton observations of the Galactic globular clusters NGC 2808 and NGC 4372
    Servillat M., Webb N.A., Barret D.
   <Astron. Astrophys., 480, 397-407 (2008)>


This table was created by the HEASARC in February 2009 based on the electronic version of Table 2 from the paper which was obtained from the CDS (their catalog J/A+A/480/397 file table2.dat).


A unique identification number for each XMM-Newton X-ray source in this table. The 5 sources located within the cluster half-mass radius are denoted as C1 through C5. while the remaining 91 sources are denoted as 1 through 92 (omitting 4, which was the confused source near the cluster core subsequently resolved into 5 4-sigma and 3 other 2.5-sigma sources).

The NGC 2808 XMM-Newton X-ray source designation, as recommended by the CDS Dictionary of Nomenclature of Celestial Objects, using the '[SWB2008] NGC 2808' prefix (Servillat, Webb, Barret (2008), NGC 2808) and the source number.

The Right Ascension of the XMM-Newton 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 XMM-Newton X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to an accuracy of 0.01 arcseconds in the original table.

The Galactic Longitude of the XMM-Newton X-ray source.

The Galactic Latitude of the XMM-Newton X-ray source.

The 1-sigma XMM-Newton X-ray source positional error, in arcseconds.

The net (background- and vignetting-corrected) 0.5-10 keV count rate of the X-ray source, in ct/s.

The RMS uncertainty in the X-ray source count rate, in ct/s.

The unabsorbed X-ray flux of the source in the 0.5 - 10 keV band, calculated from the count rate, using energy conversion factors (ECFs) in the 0.5 - 10 keV band, in erg/s/cm2. These factors were calculated in each of 3 energy bands (0.5 - 1.5, 1.5 - 3.0 and 3 - 10 keV) and for each detector by extracting an on-axis source and generating detector response files for the source (using rmfgen and arfgen SAS tasks). These response files were used to create a fake spectrum corresponding to a common model: a power law spectrum with a power-law photon index Gamma=1.7 (the mean spectrum of the detected sources) and the absorption N_H of the cluster. For NGC 2808, the absorption of 1.2 x 1021 was calculated from the reddening of optical observations (Bedin et al. 2000, A&A, 363, 159) with the relation computed by Burstein & Heiles (1978, Ap. Letts, 19, 69). Finally, ECFs were calculated by dividing the count rates of the fake spectrum by the model fluxes. The obtained fluxes were then converted to unabsorbed fluxes. This method gives a reliable estimation of the ECFs needed for the 3 energy bands. To give an idea of the errors in these values, by changing the spectral index of the model to Gamma = 2.0, the fluxes changed by 1.5%, 2%, and 8%, in the energy bands 0.5 - 1.5, 1.5 - 3.0, and 3 - 10 keV, respectively.

The RMS uncertainty in the unabsorbed 0.5 - 10 keV flux of the X-ray source, in erg/s/cm2.

The hardness ratio HR1 of the X-ray source, defined as (B2 - B1)/(B2 + B1), where B2 is the 1.5 - 3.0 keV band counts and B1 is the 0.5 - 1.5 keV band counts.

The hardness ratio HR2 of the X-ray source, defined as (B2 - B1)/(B2 + B1), where B2 is the 3.0 - 10.0 keV band counts and B1 is the 1.5 - 3.0 keV band counts.

Contact Person

Questions regarding the NGC2808XMM database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk