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Astron. Astrophys. 349, L49-L52 (1999)

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3. Observations and data analysis

Our results were obtained with the Medium Energy Concentrator Spectrometer (MECS, Boella et al. 1997) that provides images in the [FORMULA] 1.3-10 keV energy range over a nearly circular field of view with [FORMULA] radius. The MECS has a good angular resolution (50% power radius of [FORMULA] at 6 keV, on-axis) and a moderate energy resolution (FWHM [FORMULA]8.5[FORMULA]%). In order to exploit the best spatial and spectral resolution of the detector, and to avoid the artifacts due to the absorbing strongbacks of the MECS entrance window, we only considered the inner region of the field of view (radius [FORMULA]).

For the spectral analysis we have used the MECS effective area values appropriate for extended sources. These were properly derived by convolving a flat surface brightness distribution with the energy and position dependent vignetting and Point Spread Function (PSF).

All the spectra have been corrected for the the instrumental background by subtracting a spectrum obtained from MECS observations of the dark Earth. This procedure does not account for the subtraction of the cosmic X-ray background (CXB). Indeed, the subtraction of the MECS standard background obtained from high galactic latitude pointings would overestimate the CXB contribution, which is highly absorbed in the GC region. The CXB contribution has been included as a known additive component in the spectral fits with an absorbing column density of 8[FORMULA] cm-2 and spectral parameters fixed at the values determined with the MECS (photon index = 1.44, F(1 keV)=12.9 phot cm-2 s- 1 sr-1 keV-1; S. Molendi, private communication).

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999