We shall start with a simple example based on very old data, an EXOSAT observation of GX301-2. First we read in the data :

                XSPEC version: 12.5.1
        Build Date/Time: Mon Aug  3 13:17:56 2009

XSPEC12>data file1.pha

1 spectrum  in use
 
Spectral Data File: file1.pha  Spectrum 1
Net count rate (cts/s) for Spectrum:1  3.586e+01 +/- 2.466e-01 (50.6 % total)
 Assigned to Data Group 1 and Plot Group 1
  Noticed Channels:  1-64
  Telescope: EXOSAT Instrument: ME  Channel Type: PHA
  Exposure Time: 2100 sec
 Using Background File                back1.pha
  Background Exposure Time: 3320 sec
 Using Response (RMF) File            resp1.rsp for Source 

This dataset has associated background and response files which have been read in automatically because their names are given in file1.pha by the BACKFILE and RESPFILE keywords. If these keywords were not set, we could load the associated files using :

XSPEC12>back back1.pha
Net count rate (cts/s) for Spectrum:1  3.586e+01 +/- 2.466e-01 (50.6 % total)
XSPEC12>resp resp1.rsp
Response successfully loaded.