The PHA file standard includes several keywords that are used to store information about the instrument response associated with the spectrum. If these keywords are defined then the fitting engine - such as Sherpa, ISIS, or XSpec - will automatically load in the response files for you.

The relevant keywords are:

• ANCRFILE - the name of the ARF for the source.
• BACKFILE - the name of the PHA file containing the background for the source.
• RESPFILE - the name of the RMF for the source.

These keywords are set if you use a CIAO script, such as specextract, but can also be set manually by using the dmhedit tool.

To check the values:

unix% dmlist src.pi header | grep FILE
0100 BACKFILE             bgnd.pi                        String
0101 CORRFILE             none                           String
0102 RESPFILE             src.rmf                        String
0103 ANCRFILE             src.arf                        String

or

unix% dmkeypar src.pi ancrfile echo+
src.arf