In the central 8 radius extraction region the 0.1-10 keV NXB count rate is s-1 keV-1 arcmin-2. The dependence of the NXB on position within the FOV is shown in the right panel of Fig. 3. The difference in intensity between the most and least intense regions is a factor 3.1. In contrast to the standard background (left panel of Fig. 3), the NXB intensity near the center of the FOV does not show an enhancement. Within the central 5 radius, any such increase is <4% of the total intensity.
|Figure 5: The dependence of the FOV averaged LECS dark Earth count rate on time. The launch date was 1996 April 30|
Spectra of the LECS NXB are shown in Figs. 4 and 6. The overall level is approximately constant with energy with 3 discrete features superposed on a smooth increase 4 keV. These features can be modeled as narrow Gaussian emission lines at keV (at 68% confidence), 8.51 0.04 keV and 10.73 0.07 keV. The first feature shows a clear position dependence, being stronger close to the calibration sources. It is almost certainly produced by 5.9 keV characteristic X-rays from the 55Fe calibration sources that penetrate deeply into the detector before being absorbed. The detected energy is lower than the natural energy of the events, because X-rays absorbed deep within the detector produce, on average, less light than X-rays of the same energy absorbed close to the entrance window due to the different scintillation lengths in the driftless gas cell (Parmar et al. 1997b). The other 2 features do not exhibit an obvious position dependence within the FOV and may originate from fluorescent excitation of L-shell transitions in the tungsten window support structure (see Parmar et al. 1997b).
To illustrate the spectral changes associated with the decrease in count rate shown in Fig. 5, Fig. 6 shows the spectrum of the NXB before and after day 400. This shows that at energies 8 keV the long term temporal evolution of the LECS NXB is not strongly energy dependent and may be simply modeled as the change in overall normalization given above. Above 8 keV, the intensity variation with time is less marked. This may indicate that the fluorescent line features exhibit less intensity variability than the rest of the spectrum.
|Figure 6: The dependence of the rebinned LECS NXB spectrum, averaged over the entire FOV, on time. The filled circles show the spectrum obtained before day 400 and the open circles after day 400. The lower panel shows the ratio of the two spectra|
The geomagnetic rigidity is a measure of the minimum momentum required by a cosmic particle to penetrate the Earth's magnetic field down to the position of the satellite. Due to the almost circular, low inclination BeppoSAX orbit, the variation in rigidity around the BeppoSAX orbit is less than is typical for low-Earth orbiting spacecraft such as ASCA which has an orbital inclination of 31 and experiences rigidities between 6-14 GeV c-1. In the case of BeppoSAX, the rigidity typically varies from 10 to 16 GeV c-1 around the orbit. In order to investigate the dependence of the NXB spectrum on rigidity, spectra were accumulated for intervals when the rigidity was and >13 GeV c-1. The two spectra have almost identical shapes, with the spectrum accumulated when the rigidity was 13 GeV c-1 having an overall normalization % higher. This means that for all but the shortest observations temporal averaging will ensure that the dependence of the NXB on geomagnetic rigidity can be ignored.
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