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.
If the file contains multiple spectra then the appropriate one can be specified by appending {N} to the end of the filename, where N is the row number of the spectrum in the file. Alternatively, the spectrum can be specified by {column_name=value} where column_name is the name of a string column in the table.
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 and arf files). 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. An error term is calculated
using Poisson statistics and any systematic error indicated in the
file.