Abstract: It has been over a decade since the first paper was published containing results determined using the general X-ray spectral-fitting program XSPEC. Since then XSPEC has become the most widely used program for this purpose, being the de facto standard for the ROSAT and the de jure standard for the ASCA and XTE satellites. Probably the most important features of XSPEC are the large number of theoretical models available and the facilities for adding new models.
Credit: Arnaud, Keith; Dorman, Ben; Gordon, Craig
Site: https://heasarc.gsfc.nasa.gov/docs/xana ... index.html