OGIP Spectral Files: .pha and .pi

These file formats were defined by Arnaud, George, and Tennant (1992), and are used extensively by the X-ray spectral analysis packages XSPEC and Sherpa^3.1. In its most basic form, this spectral format contains only the number of counts in each detector channel. The energies or wavelengths associated with each channel are not listed. As a result, profit requires a second file known as the ``response'' file (usually ending in .rmf or .rsp) to be designated. This file contains the energy bounds (``EBOUNDS'') for each channel. In some cases, the spectral file internally lists the appropriate response file, and in this case, profit will automatically read the response file.

An extension to the OGIP standard defined the ``Type II PHA'' file, allowing one file to contain multiple spectra. This format is the default for Chandra HETG and LETG grating data. When profit reads a Type II PHA file, it simply lists each of the spectra (by number) in the Spectrum menu bar. The user is then responsible for loading the appropriate response. In Chandra Type II files, the number of spectra depends upon the grating and detector system used, as well as how the data were processed. Table 3.1 shows the default association for the three standard grating/detector combinations, but the user should be warned that non-standard data reduction can change these.

We note that at the present time the Chandra archive does not include the response files; therefore, the user is expected to create specific response files for each dataset. As an alternative, generic grating response files can be downloaded from http://asc.harvard.edu/caldb/prop_plan/grating/index.html, although these may not be appropriate for grating observations done in non-standard modes.

Table 3.1: Chandra grating Type II PHA datafiles and associated responses.

HETG/ACIS		LETG/ACIS		LETG/HRC
#	Response	#	Response	#	Response
1,6	3rd order HEG	1,6	3rd order LEG	1,2	All orders LEG
2,5	2nd order HEG	2,5	2nd order LEG
3,4	1st order HEG	3,4	1st order LEG
7,12	3rd order MEG
8,11	2nd order MEG
9,10	1st order MEG