STScI Logo

xspec_interface xray



xspec_interface -- how to convert data into XSPEC PHA files


Versions of XSPEC after 8.2 can fit data in either SF (Standard Format) or FITS format. Two files must be specified to fit data in XSPEC, and both must be generated outside of XSPEC. One file is the spectral table file which contains the net counts and errors in each energy bin. The second file is the response matrix (actually the convolution of the redistribution matrix and the effective area appropriate for the location on the detector of the extracted source photons).

In PROS 2.3 (released 1993 December) the output from qpspec was changed to conform to the RDF FITS standards. After extracting a spectrum with qpspec, the data must be converted into pre-RDF format with the task downspecrdf. This task combines the 3 RDF spectral table files (extensions,, into a single pre-RDF spectral table file (extension Then, outside of IRAF, the task proscon should be used to generate the SF format spectral table file (extension .pha) and SF format response matrix (extension .rsp). Proscon uses the source off-axis histogram to generate a response matrix with the correct effective area for the source position on the detector.

Note: The output from the PROS 2.3 version of downspecrdf was not always compatible with proscon. An updated version of proscon is now available via anonymous ftp from the GSFC node "" in the subdirectory


For on-axis sources, the output from qpspec can be converted directly into an OGIP fits format pha file with the tasks stsdas.stwfits and ftools.caltools.stw2pha. As of FTOOLS 2.8, however, there is not a way generate a response matrix for an off-axis source.


To convert spectral table files into XSPEC PHA files execute the following commands:

In iraf:

	xs> qpspec
	xs> downspecrdf

Outside of iraf:

	) proscon


Prior to XSPEC 8.4, the Fe abundance in the Raymond model was given relative to the solar value and not the cosmic value as in PROS. Direct comparisons between PROS and XSPEC can be done by setting:

	XSPEC Fe abundance = 1.26 PROS Fe abundance

In versions of XSPEC starting with 8.4, the Fe abundance is measured relative to the cosmic value.

Galactic absorption in both XSPEC and PROS is approximated as a piecewise cubic polynomial. Wabs in XSPEC and abs in PROS use identical energy boundaries and coefficients for the polynomials. PROS calculates the absorption based on the central energy in each PI bin. XSPEC performs a different numerical approximation when there is an edge within a given PI energy bin. The difference between the two models is only noticeable in soft, highly absorbed spectra around the carbon edge. The difference is only about 3% which is less than the 10% systematic errors in the PSPC response matrix at the carbon edge.

Search Form · STSDAS