To obtain the observed spectrum (C(I)), for a given observation, XSPEC uses two files:
the data file, and the background file (for FITS file format see Arnaud, George & Tennant 1992
). The data file tells XSPEC how many total photon
counts were detected by the instrument in a given channel. XSPEC then uses the background file
to derive a background-subtracted C(I) in units of counts per second.
The background is scaled to the data by the ratio of the BACKSCAL keywords in the data and
background files. It also is scaled for exposure times (EXPOSURE keyword).
Having done this, XSPEC now has an observed spectrum to which the model spectrum may be fit.