The input PHA files required by XSPEC Versions prior to 8.2
are in the so-called SF format2.
This is a non-portable (sequential access) binary format which stores the
required data as a number of “packages” within the file.
The FITS format must contain all the information contained in the mandatory
SF “packages”. This information comprises:
Additional optional information can also be used by XSPEC. This
- The data plus optional errors and systematics. The data is
either in counts in which case Poisson errors will be applied by XSPEC
(and optional systematic errors can be given) or in count/s in
which case the file must also include errors (and optional systematic
errors). The systematic errors should be expressed as a fraction of
- File information. This comprises a detector identification string,
the number of channels in the PHA spectrum, the integration
time, the area
scaling factor, the background scaling factor (for an imaging
instrument this should be the area on the sky over which the spectrum
is accumulated), the correction scaling factor.
- Associated file information. Also required are pointers to the
default background, correction, redistribution matrix (RMF),
and ancillary response (ARF) files.
- Grouping information. This specifies any "raw" detector
channels that should be
ignored by default and also includes any user-specified binning of the
data to be performed. In the SF files the actual stored data is binned
and the grouping card describes how this was done so that XSPEC can
bin up any background and response in the same way. In the FITS format
this is modified so that the data is always stored in unbinned
mode and the grouping card indicates to XSPEC how data should be
binned up when it is read in.
- Filter information. This consists of a set of character strings
which are used to specify how the data was selected. This might
include time ranges, spatial region specifiers, phase specifiers
This information is used in XSPEC to perform joint spectral/spatial
and spectral/temporal analysis.
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Last modified: Thursday, 21-Oct-2021 12:50:38 EDT