In the case of the LECS, we present 3 ways of subtracting the background. An estimate of the systematic uncertainty associated with background subtraction may be obtained by comparing results obtained using the different methods.
This is the simplest approach since the standard background includes the contribution from the high (>) galactic latitude CXB and the NXB. Since the standard background has a longer accumulation time (558.6 ks) than individual BeppoSAX observations, this method does not usually add significant noise. Since the standard background varies with position within the FOV (Fig. 3), the background spectrum should be accumulated using the same extraction region as the source. Note that this technique will underestimate the low-energy background for pointings in directions with bright Galactic emission such as the North Polar Spur (see Sect. 4).
This technique has the following steps:
-CIRCLE(128.0,128.0,80.0) -CIRCLE(128.0,128.0,62.85) -BOX(64.0,192.0,128.0,128.0) -BOX(192.0,64.0,128.0,128.0).
ROSAT PSPC all-sky survey diffuse background maps are available in 7 energy bands with a pixel size of covering 98% of the sky. The effects of discrete source counts, non X-ray contamination and X-rays of solar system origin have been eliminated to the greatest possible extent. The contributions of all the sources listed in Table 4 have been excluded. These maps can be used to predict the variation with pointing position of the LECS background. The PSPC count rates closest to the centers of each of the fields included in the LECS standard background were first determined. The mean, LECS exposure weighted, PSPC 0.1-2.5 keV count rate is s-1 arcmin-2.
In order to understand how best to model the PSPC data from different sky positions, 7 channel PSPC spectra were determined for the fields given in Tables 2 and 4. These were then fit with a number of different models including power-laws, thermal bremsstrahlungs, broken power-laws with the hard component fixed at 1.4 (the value expected above 1 keV), and power-laws with ,together with thermal components. In all cases low-energy absorption was included. It was found that (1) the power-law and (2) the fixed power-law together with a thermal component give consistently the best fits for almost all the fields. These two models are therefore used in the background analysis except for the Gal cent-3 field, where a broken power-law, with hard index fixed at 1.4 is preferred.
In order to use this technique to predict the LECS background spectrum at a particular pointing position, the following steps are necessary:
The LECS CXB spectrum is discussed in a companion paper (Parmar et al 1999). In the 0.1-7.0 keV energy range it is well modeled by an absorbed power-law together with two solar abundance thermal bremsstrahlung components (the MEKAL model in XSPEC). When this model is fit to the exposure weighted PSPC spectrum corresponding to the standard background, the best-fit results are a power-law of 1.48 with normalization of photon cm-2 s-1 keV-1, a hard bremsstrahlung component temperature (kT) of 0.80 keV with normalization of photon cm-5, and a soft bremsstrahlung component kT of 0.159 keV with normalization of photon cm-5. For both absorbed models, equivalent to atom cm-2 is required. These fit parameters should be used to generate the predicted standard LECS background spectrum.
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