The first version of XSPEC was written in 1983 at the Institute of Astronomy, Cambridge, under VAX/VMS by Rick Shafer. It was written to perform spectral analysis of data from the ESA EXOSAT X-ray observatory, which was launched that year. XSPEC is a descendant of a series of spectral-fitting programs written at NASA/Goddard Space Flight Center for the OSO-8, HEAO-1 and Einstein Observatory missions. The initial design was the fruit of many discussions between Rick Shafer and Andy Szymkowiak.
Rick Shafer subsequently joined the EXOSAT group, where he enhanced the VAX/VMS version in collaboration with Frank Haberl. Meanwhile, Keith Arnaud ported XSPEC to a Sun/UNIX operating system. The two implementations of XSPEC diverged for several years until a determined and coordinated effort was made to recover a single version. The results of that effort was XSPECv6, described in the first edition of this manual. Subsequent editions have covered later versions of the program.
In recent years XSPEC has become the de facto standard for X-ray spectroscopic analysis for the ROSAT mission and the de jure standard for the BBXRT, ASCA, and XTE missions. The HEASARC, which has supported the developement of XSPEC since 1990, intends to use XSPEC for future missions as well as for those whose data are being resurrected.