If a particular file is not found or cannot be opened for input for some reason, then the user is prompted for a replacement file name. An <EOF> at this point is equivalent to typing n one.
The default extension for all files is .pha, so all other extensions, (e.g. .fak) must be entered explicitly. The default directory is the current user directory when XSPEC is invoked.
When a new file is input, by default all its PHA channels are considered good channels for fitting and plotting purposes (see the ignore and notice commands), unless the file is replacing a previously-input file that had exactly the same number of total PHA channels, in which case the particular channels noticed are not modified.
The individual spectral data files are created outside of XSPEC by
detector-specific software. They are organized as XSPEC data files, but
often referred to as PHA files. The PHA file contains such information as
integration time, detector effective area, and a scaling factor that estimates
the expected size of the internal background. The data file also contains the
names of the default files to be used for background subtraction and for the
detector sensitivity versus incident photon energy (the response file). A data
file has the total observed counts for a number of channels and a factor for
the size of any systematic error. Each channel is converted to a
count-rate-per-unit area (assumed cm
). The default background file is used
for background subtraction, and an error term is calculated using Poisson
statistics and any systematic error indicated in the file.