## PHA keywords

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 or the header-editing capabilities of prism.

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