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

58 CHAPTER 6. BAT ANALYSIS ISSUES
was added in version 1.50, and was meant to remove from the output catalog sources with bad
DETECT STATUS values. However, new sources were also removed by the CFITSIO library
routine when they shouldn’t have been.
The workaround is to set “keepbadsources=YES” and filter the output catalog manually based
on DETECT STATUS.
6.2.5 batcelldetect: the correct point spread function is gaussian
Task: batcelldetect
Version: 1.11-1.59
What builds: HEASoft 6.2 and earlier
Problem: Default point spread function of PYRAMID is incorrect
Status: Closed
Updated: 17 Jul 2007
NOTE: This problem was fixed in HEASoft 6.3 (task version 1.60 and greater).
Fluxes derived from batcelldetect are based on a fit of a point spread function (PSF) to the sky
image intensities. In particular, the reported flux is the intensity at the center of the PSF. As one
might expect, the flux depends on fitting the correct PSF model to the data.
Based on a number of incorrect assumptions, the BAT team reported that the PSF was a
truncated pyramidal frustum. However, this is incorrect. In fact, the PSF is very nearly a gaussian
function (with full-width half-maximum of 22 arcmin). This PSF can be selected by using the
psfshape=GAUSSIAN option to batcelldetect (the default is psfshape=PYRAMID).
Using the incorrect frustum function will result in fluxes that are too high by 4.0% compared
to the true flux. The signal to noise ratios are high by about 3.5%. For the most part, this effect
is only significant for the brightest sources, where the statistical errors are smaller than 4%.
Note that this problem applies only to fluxes from batcelldetect. It doesn’t affect mask-weighted
light curves and spectra made from batbinevt.
6.2.6 Analysis: Passive materials distort the off-axis counts/rates
Task: All flux extraction (batfftimage/batmaskwtevt/batmaskwtimg/batbinevt)
Version: All versions
What builds: All builds
Problem: Passive materials introduce errors in mask-weighted counts/rates
Status: Corrective Procedure (HEASoft 6.0.3)
Updated: 15 Oct 2005
The BAT image system works by forming shadow patterns cast by the mask onto the detector
array. The mask is made from lead tiles. However, the mask support structure also contains
significant absorbing materials. The edge of the support structure is a particular problem because
(a) extra absorbing materials (e.g. epoxy) were applied, and (b) this material rises up above the
plane of the mask. The net result is that for sensitive imaging and spectroscopy of off-axis sources,
the full mask aperture cannot be used.
The BAT team has provided a new set of aperture files in CALDB, tied to the HEASoft 6.0.3
release. These apertures have been reduced in size to appropriately block out the shadows of most
of the absorbing material. However, this does reduce solid angle sky coverage by 5-10%.
The aperture files are now divided into two classes: