The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

26 CHAPTER 4. INTRODUCTION TO BAT ANALYSIS
... per ... *illuminated* detector ... (correction ’pcode’)
The ‘pcode’ correction normalizes the counts by the number of illuminated detectors. Since the
BAT is a wide-field instrument it is possible, indeed likely, that sources will be at the edge of the
field of view. This means that only a fraction of the detector array is illuminated by through mask,
i.e. the source is partially coded, and thus will produce fewer counts. This correction normalizes
the partial coding fraction to unity for all sources.
... per *fully illuminated* detector ... (correction ’maskwt’)
Even when detectors are illuminated by a source, most detectors are not 100% illuminated. In
general, there is a distribution of detector illumination fractions, ranging from 0% (fully shadowed
by the mask) to 100% (fully illuminated by the mask). The mask weighting technique used by the
BAT software combines all events to produce background-subtracted counts in a “mean” fractionally
illuminated detector. The ‘maskwt’ correction renormalizes the counts to be those received in
a fully illuminated detector. This accounts for both the open fraction of the mask (50% for the
BAT), plus the per-detector partial illumination.
The ‘maskwt’ correction should be distinguished from the partial coding correction. The partial
coding correction adjusts for the fractional number of detectors that a source could illuminate
through the mask, regardless of which mask cells are blocked or open. The ‘maskwt’ correction
adjusts for the partial illumination of each individual detector.
... for an equivalent on-axis source* (correction ’flatfield’)
Off-axis sources have a different count rate because the projected area of the BAT detectors is
changed by projection effects. Generally, the effective area is reduced off-axis because of the “cosine
effect.” However, the effective area increases slightly for mild off-axis angles because detector sides
are illuminated. The ‘flatfield’ correction adjusts for these effects, to create a count rate for an
effective on-axis source. In this manner, it will be more practical to compare detections of the same
source at different positions in the field of view.
It should be noted that this correction is primarily due to geometric effects. It is also understood
that off-axis illumination changes the detector response in non-subtle ways, which is not corrected
for by the ‘flatfield’ correction.
4.7.2 Converting to a counts per area
The area of a single BAT detector is 0.16 cm 2 . Thus, the conversion between “per unit area” and
”per fully illuminated detector” is:
counts_cm2 = counts_fully_illuminated_det / 0.16