XSPEC is distributed and maintained under the aegis of the GSFC High
Energy Astrophysics Science Archival Research Center (HEASARC). It can
be downloaded as part of HEAsoft from
http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/download.html
XSPEC is available either as binaries or source. We recommend downloading the source and compiling locally to avoid potential system library conflicts and allow installation of any patches we release. If you have any problems compiling or running XSPEC please e-mail us at xspec12@athena.gsfc.nasa.gov
The XSPEC Web page comprises links to the anonymous ftp site, a Web
version of the manual, and several useful documents, including a list
of known bugs. The Web address is
https://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/index.html
with the list of issues and available patches at
https://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/bugs.html
and additional models which can be added at
https://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/newmodels.html
Further useful information can be found on the XSPEC Wiki at
https://asd.gsfc.nasa.gov/XSPECwiki/XSPECPage
and via the XSPEC Facebook group at
https://www.facebook.com/groups/320119452570/