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:

  1. 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 the data.
  2. 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.
  3. Associated file information. Also required are pointers to the default background, correction, redistribution matrix (RMF), and ancillary response (ARF) files.
Additional optional information can also be used by XSPEC. This includes:
  1. 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.
  2. 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 etc. 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