The energy dependence of "spill-over'' was investigated by subtracting
the count rates obtained with blank fields from each of the semi-annuli
spectra and dividing the remaining counts by those of the source with
the standard background subtracted.
Figure 8 shows
a polynomial fit (Table 3)
to the ratio derived in this way as a function of energy.
There are at least two effects which produce these additional counts in the
background semi-annuli.
At low-energies "spill-over'' is dominated by the poor detector
spatial resolution (), while
at high-energies the mirror scattering wings dominate.
![]() |
Figure 8: Ratio of additional counts in the background semi-annuli divided by source counts as a function of energy. The solid line shows the fifth-order polynomial fit given in Table 3 |
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The blank field exposures listed in Table 2 were
also used to derive the offset correction factors which are shown
in Fig. 9.
Above 4 keV, where there are few sky background counts, the correction
factors were derived by extrapolating results obtained from a Crab Nebula
observation at an offset of 11.Multiplication of the
NXB subtracted spectrum in the semi-annuli by these factors gives
the predicted background spectrum in the central 8
radius source
extraction region when the NXB is added.
These correction factors are not
simply the mirror vignetting function given in Conti et al. (1994)
since, other factors such as an increase in PMT
noise due to the lower signal levels off-axis and the position
dependency of singularly reflected X-rays may contribute.
The correction factors
do not change significantly within the range of nominal target locations
within the FOV of
2
(see Sect. 3).
Correction factors have also been determined for extraction radii
of 4
and 6
. These may be obtained by scaling the values
in Fig. 9, which are for an extraction radius of
8
, by the ratio of extraction region areas.
The ability to accurately derive these correction factors is limited
by the low number of counts in the standard background exposure.
As more LECS background fields become available, these correction
factors will be updated.
![]() |
Figure 9:
Correction factors to be applied to a sky spectrum obtained
from the background semi-annuli to give the predicted sky
background in the standard 8![]() |
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