XSPEC is a command-driven, interactive, X-ray spectral-fitting program, designed to be completely detector-independent so that it can be used for any spectrometer. XSPEC has been used to analyze data from HEAO-1 A2, Einstein Observatory, EXOSAT, Ginga, ROSAT, BBXRT, ASCA, CGRO, IUE, RXTE, Chandra, XMM-Newton, Integral/SPI, Swift and Suzaku. There now over 1000 papers listed on ADS which cite the Arnaud 1996 XSPEC reference.
This manual describes XSPEC v12.5, which is available on Linux (gcc 3.2.2 and later), MacOSX/Darwin (gcc 3.3 and later) , Solaris (2.6-9; WS6.0 and later). New users are advised to read Chapter 2, which introduces spectral fitting and the XSPEC approach, Chapter 3, which gives an overview of the program commands, and Chapter 4, which contains walkthroughs of XSPEC sessions. They should then experiment with XSPEC and, if necessary, look up individual commands in Chapter 5, or descriptions of the spectral models in use, in Chapter 6.
The User Interface for XSPEC, which employs a tcl scripting shell and the XSPEC parser are described in Appendix A. Users possessing X-ray spectra with small numbers of counts per bin are referred to Appendix B, which describes the C-statistic option. Users interested in adding their own models can read how to do so in Appendix C. Appendix D describes the PLT plotting package which XSPEC currently uses for graphical output. Some of the tools (FTOOLS, fortran programs, scripts) that can operate on XSPEC files are listed in Appendix E. The XSPEC model library can be linked into other programs following the instructions in Appendix F. Appendix G describes how to use your own proposal distribution for Markov Chain Monte Carlo. Finally, Appendix H includes some notes on the changes between XSPEC v11 and v12.
· Gain parameters can now be used in the error, freeze, newpar, thaw, and untie commands by prefixing the command name with the letter ‘r’ (for “response parameter”, the more general category to which gain parameters belong). Steppar can now also handle gain parameters. Gain parameters can be displayed either with “show parameter” or the new “show rparameter” option.
· The gain command syntax has changed when using multiple sources. To better conform with the rest of XSPEC, it now requires <source number>:<spectrum number> rather than the reverse.
· Gain parameter limit values can be stored in response files, using the keywords GSLOP_MIN, GSLOP_MAX, GOFFS_MIN, and GOFFS_MAX.
· All input and output data filenames can now include CFITSIO/FTOOLS extended-syntax for specifying particular HDUs. As a result, XSPEC can now handle files which contain spectra, ARFs and RMFs in multiple extensions.
· Partial derivative calculations during fitting can now be performed numerically rather than with an approximated analytical expression. This option is chosen in the Xspec.init initialization file.
· If a new minimum is found during a steppar run, steppar now prompts the user for acceptance of the new values. Also the delta statistic column of a steppar run is now obtainable with the tclout steppar delstat option.
· The output warning message has been improved in the case where Levenberg-Marquardt fitting runs into a zero diagonal element in the second derivative matrix. Similarly, the more frequent pegged-parameter messages (due to running into hard limits) is now output at higher chatter levels only.
· All calls to the xanlib dynamic memory allocation function udmget have been removed from the Fortran models in XSPEC’s models library. The relevant code has been converted to C++. If a user’s local models library still requires the udmget code, they’ll need to run initpackage with the new -udmget option.
· Additional enhancements previously released as patches to 12.5.0:
· Setplot wave x-axis units can be toggled from Hz to angstroms through WAVE_PLOT_UNITS entry in Xspec.init file.
· New tclout gain and sigma options.
· New xs_getVersion function available for those linking their own programs to the XSPEC models library.
· The show parameters option can now take a range of parameters for displaying subsets.
· All bug fixes to v12.5.0 released as patches a - an are included in v12.5.1. In addition the following problems have been corrected.
· After running the ARF command, any gain previously applied to the associated RMF will be removed. Previously it was erroneously applying the gain to the new ARF.
· Additional header file inclusions needed in code files to compile with g++-4.4.0
· Extra line-feed characters removed from Ascii text files in the modelData directory. These were causing problems on Solaris 10 w/f90.
· The nthcomp model’s internal arrays were hardcoded to a maximum size of 5000 energy bins. The size is now dynamically allocated. (This also affects the diskir model.)
· A Levenberg-Marquardt fit now immediately stops if the fit statistic becomes NaN due to an erroneous model calculation.
· C++-style comments have been removed from xsFortran.h for the benefit of users compiling their own C programs with the models library.
· Plotting fix for case where “setplot area” is selected and no models are currently loaded.
· Model parsing fix for case of nested parentheses with no ‘+’ operator, ie. A(B(C*D)).
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
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
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
with the list of issues and available patches at
http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/bugs.html
and additional models which can be added at
http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/newmodels.html
Further useful information can be found on the XSPEC Wiki at
https://astrophysics.gsfc.nasa.gov/XSPECwiki/XSPECPage
and the xspector blog at
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 EO 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 RXTE missions. It is now in extensive use for all current X-ray and gamma-ray missions. An extensive re-engineering effort was started in the fall of 1998 to improve long-term maintainability, allow significant new features to be added, and support the analysis of spectra from coded-mask instruments.
This project would not have been possible without ignoring the advice of far more people than can be mentioned here. We would like to thank all our colleagues for their suggestions, bug reports, and (especially) source code. The GSFC X-ray astronomy group are particularly thanked for their patience exhibited while functioning as the beta test site for new releases. The initial development of XSPEC was funded by a Royal Society grant to Prof. Andy Fabian and subsequent development was partially funded by the European Space Agency's EXOSAT project and is now funded by the HEASARC at NASA/GSFC.
Dorman, B., Arnaud, K. A., and Gordon, C. A. XSPEC12: Object-Oriented X-Ray Data Analysis, AAS HEAD meeting No. 35, #22.10