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

6.3. OLD ISSUES 67
1. Sources will appear to be at the wrong locations;
2. Mask weighted fluxes for known sources will be lower, or zero,
• since the known celestial position will translate to the incorrect detector position. This
causes the mask weighting function to sample the PSF off-center, and thus reduces the
response.
Users with pre-launch files should download a revised set of CALDB files. (20041120 version 1
or later).
6.3.13 batbinevt: Incorrect EXPOSURE Keyword
Task: batbinevt
Version: 1.7 and earlier
What Builds: Swift 1.1 and earlier (Build 11 and earlier)
Problem: Incorrect EXPOSURE keyword written to spectral files
Status: Closed (Swift 1.2)
The batbinevt task makes spectra. It writes “Type II” files which can have multiple spectra
per file. It writes an EXPOSURE column to record the exposure for each spectrum.
However, the input event or survey file also has an EXPOSURE keyword, which was not being
overwritten. Thus it was possible to have conflicting exposure values. Unfortunately, XPSEC uses
the keyword (incorrect) value instead of the column (correct) value.
This bug is fixed in Swift 1.2.
Workaround. Remove the EXPOSURE keyword from the spectral file. Since the EXPOSURE
column is still present, XSPEC will still work, and should report the correct fluxes and fluences.
Commands to remove the EXPOSURE keyword from myfile.pha:
fthedit myfile.pha EXPOSURE delete
6.3.14 batcelldetect: Incorrect Error Bars
Task: batcelldetect
Version: 1.09 and earlier
What Builds: Swift 1.1 and earlier
Problem: Error bars reported by batcelldetect are too small by a factor of ∼4
Status: Corrected for Swift 1.2
Discussion: batcelldetect performs a PSF fit to detected sources in an input image. This fit is
done with a least-squares routine. Errors are estimated by using the formal statistical error (i.e.
covariance matrix). The problem is that the error bars were too small by a factor of ∼4 compared
to the “true” values based on population analysis. This turned out to be because the images were
oversampled by a factor of 2 in each dimension, which affects the statistics. batcelldetect attempted
to account for the oversampling, but did it incorrectly in versions 1.09 and earlier.