Up: BeppoSAX Low-Energy Concentrator Spectrometer
In order to study the properties of the LECS background, data were extracted
from 350 BeppoSAX Final Observation Tapes, corresponding
to 170 individual observations.
These data were processed using the standard LECS processing pipeline
described below and cleaned event lists for the intervals when
the instrument was observing both the dark Earth and the
sky in the nominal operating configuration extracted.
Observations with <100 counts were excluded. This left
total sky and dark Earth exposures of 2847 ks and 498.6 ks, respectively.
Table 1 gives the contents of the LECS telemetry
for each detected event. The standard LECS data processing package
(SAXLEDAS 1.8.0) performs the following steps to produce cleaned,
linearized event lists:
- 1.
- Conversion of RAWX and RAWY into linear coordinates referred to
as DETX and DETY using the algorithm given in Parmar et al. (1997b).
- 2.
- Determination of the overall instrument gain (which is temperature
dependent), using the mean energy of the two calibration sources as
a reference.
- 3.
- Conversion of Pulse height channels to Pulse Invariant
(PI) channels which have a fixed relation to energy. This
conversion takes into
account the overall gain and a FOV position dependent gain correction.
- 4.
- Selection of intervals with nominal instrument settings (such
as high voltage settings), and separation into events that are >4
from the Earth's limb (taken to be good on-target intervals), and those
where the dark Earth occults the FOV.
- 5.
- Removal of background events using the VETO and BL signals. The
VETO signal is the ratio between the light measured by the central PMT anode
and the sum of that measured by the surrounding anodes.
It therefore has a high value
for on-axis X-ray sources and lower values for events occurring further
off-axis. In SAXLEDAS 1.8.0 a sliding cell VETO window is applied
to the data. The BL signal is a measure of the duration of the light
flash produced following absorption in the gas cell. It depends on the
penetration depth of an event into the cell, which is energy dependent.
In SAXLEDAS 1.8.0 events are selected if their PI and BL values
are within a set of 62 pre-defined boxes.
The standard LECS data analysis technique is to extract counts
within a radius of 8' (corresponding to 35 RAW pixels; see
Fig. 1)
from the source centroid using the cleaned event lists produced
by SAXLEDAS. This large radius ensures
that 95% of the 0.28 keV X-rays are included in the analysis.
For absorbed, or very faint sources, extraction radii of 4
or
6
may be appropriate.
The BeppoSAX pointing accuracy is such that most target sources are located
within
2' of the nominal position of RAWX, RAWY = 131, 124.
Table 1:
LECS telemetry contents
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