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4 Standard background

  The usual method for LECS background subtraction is to subtract the counts obtained in the same detector region using a standard background exposure. This consists of the sum of the blank field exposures listed in Table 2. The LECS data for these fields were processed using SAXLEDAS 1.8.0. No point sources are present in the individual fields with 0.1-2.0 keV fluxes >$1.7 \ 10^{-13}$ erg s-1, or 2-10 keV fluxes of >$9.7 \ 10^{-14}$ erg s-1. Searches were also made of X-ray catalogs such as the ROSAT Bright Source Catalog (Voges et al. 1996) to check for the presence of contaminating point sources in, and a couple of degrees outside the FOV. Note that the Gal cent-2 and -3 fields include the complex region of sky around the Draco nebula (e.g., Kerp 1994; Moritz et al. 1998). A 22.1 ks BeppoSAX background exposure on 1996 August 11 at RA ${\rm = 14^h 42^m 27\hbox{$.\!\!^{\rm s}$}1}$, Dec $= +19^\circ44\hbox{$^\prime$}10\hbox{$^{\prime\prime}$}$was not included in the standard background since it is in the direction of the North Polar Spur. Examination of the LECS spectrum showed that it has significantly more low-energy flux than the fields listed in Table 2. The remaining exposure time is 558.6 ks.

The standard background 0.1-10.0 keV count rate in the central 8$^\prime$ radius source extraction region is $1.0 \ 10^{-5}$ s-1 keV-1 arcmin-2. The dependence of the standard background count rate on position within the FOV is shown in the left panel of Fig. 3. This image has been smoothed using a Gaussian filter with a $\sigma$ of 4 RAW pixels. The difference in intensity between the most and least intense regions is a factor 1.8. The X-ray background appears brighter close to the center of the image due primarily to mirror vignetting. Note that the LECS mirror axis is offset by 3$^\prime$ from the center of the FOV due to an alignment error during integration. The two bright arcs are located radially inwards from the 55Fe calibration sources and result from 5.9 keV characteristic X-rays that are absorbed some distance from their origin.

\psfig {figure=ds1657f3a.eps,width=8.0cm,angle=-90}

\psfig {figure=ds1657f3b.eps,width=8.8cm,angle=-90}

 }}\end{figure} Figure 3: The dependence of the LECS standard background (left panel) and NXB (right panel) on RAWX, RAWY position within the FOV. Gaussian smoothing filters with a $\sigma$ of 4 pixels have been applied

Figure 4 illustrates the overall shape of the standard background spectrum obtained in the central 8$^\prime$ of the FOV. The Non X-ray background (NXB) obtained from the same region of the detector is also shown (see Sect. 5). The difference between the two is the contribution of the cosmic X-ray background (CXB), spectral fits to which are presented in a companion paper (Parmar et al. 1999).

\psfig {figure=ds1657f4.eps,angle=-90,width=8.5cm}
}\end{figure} Figure 4: Rebinned spectrum of the standard background (filled circles) and the NXB (open circles) in the central 8$^\prime$ radius of the FOV

Table 2: LECS observations used to create the standard background spectrum. A target name including "sec'' or "secondary'' refers to the prime WFC target. N is the number of individual pointings. ${ N_{\rm gal}}$is the line of sight absorption in units of 1020  atom cm-2 (Dickey & Lockman 1990)

 Target & \multicolumn{2...
 ...1 01 01 &
89.9 & +30.6 & 102.2 & 3.5 \\ \noalign{\smallskip}

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