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

5.7. SPECTRUM AND RESPONSE MATRIX 45
5.7.6 Generate a Response Matrix
A response matrix should be created for each spectrum where the GRB is at a different position in
the BAT field of view. For example, one response matrix should be made for data taken before any
slew, and one after any slew. Any spectra made during the slew should have their own response
matrix.
batdrmgen flare.pha flare.rsp NONE
5.7.7 Important: Spectrum of a Source During a Slew
It is often the case that a gamma-ray burst source is moving in the BAT field of view. This is
because the spacecraft often slews to the position of the GRB before the burst emission is has ended.
The BAT spectral response changes with position, so you should not use one response matrix for
the entire burst. For a source with a power law spectrum, a position error of 25 degrees introduces
an error of about 0.1 in the photon index, and about 10% in total flux. The spectral distortion
effect will be more pronounced if the slew is a longer distance, or if the GRB has strong emission
peaks separated by a large time and/or distance. The distortion will be less if the GRB emission
occurs mostly during a pointing, and of course there will be no distortion if the burst emission is
completely confined to a pointing.
You should break long bursts which overlap slews into smaller segments corresponding to
whether the spacecraft is moving or not. Generally speaking, you should make a response matrix
whenever the position of the source changes by more than 15 degrees (or smaller increments
if you wish). One straightforward way to do this is to use the position columns in the auxiliary
ray tracing file. Look for the columns named IMX and IMY, which contain the source position in
the BAT field of view, relative to the spacecraft pointing direction. The values are tangent plane
coordinates, so tan(15 deg) ∼ 0.27. Thus, you should break a spectrum into segments where the
source moves no more than 0.25 in the IMX or IMY coordinate.
The SDC should make “pre-slew” and “post-slew” response matrices. These generally should
correspond to the pointings during the BAT trigger, and the following pointing after the slew,
respectively. These matrices will be useful if the burst primarily overlaps either the pre-slew or
post-slew pointings.
5.7.8 Analyze Spectrum in XSPEC
You can now load this spectrum within the spectral fitting program XSPEC. Please be aware that
you should ignore data below 14 keV and above 195 keV. Since the BAT mask becomes transparent
around 150 keV, the BAT team recommends fitting spectra in the 15-150 keV range.
Common Spectral Models
The following models are commonly used in fitting of BAT spectra.
powerlaw/pegpwrlw The simplest model. Powerlaw is normalized by its differential spectrum,
and pegpwrlw is normalized by total flux. NOTE: since the powerlaw model is normalized at
1 keV, it can lead to spurious statistical correlations between parameters in the BAT 15-150
keV band; pegpwrlw is preferred.