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, 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.
· New models:
· cpflux - a variant of cflux for photon flux.
· heilin - Voigt absorption profiles for the HeI series
· lyman - Voigt absorption profiles for the HI or HeII series
· zbabs - EUV ISM attenuation
· A new statistic, pgstat, has been added for the case of Poisson-distributed data with a Gaussian-distributed background. The whittle statistic can now be used when fitting averaged power density functions by appending an integer (so eg whittle5 is the statistic to use when fitting a pdf constructed by averaging those from 5 observations).
· The old CERN Minuit library, which is used for the migrad, minim, monte, and simplex fitting methods and the improve command, has been replaced by the new version. The minim and monte methods are no longer supported and the new version does not include an improve command. The output from the migrad and simplex fitting methods now looks the same as that from the leven method. Note however that the rules for when to write intermediate fit results are not directly comparable so do not provide a measure of the relative speed of the methods.
· Fakeit now has a 'nowrite' option to generate fake spectra without producing output files. This is also now available in the multifake.tcl script command.
· Parallel processing capability has been added to the steppar command and can be invoked using the parallel command.
· Markov Chain Monte Carlo (the chain command) now uses the Goodman-Weare algorithm by default. Previously the default was Metropolis-Hastings.
· After a chain run, the best-fit parameters and statistic are now displayed with "chain info", and are available through the "tclout chain" option.
· The default atom_db version used in apec models may now be modified with the ATOMDB_VERSION keyword in the user's Xspec.init file.
· Steppar now has a 'delta' option for performing grids centered on the best-fit parameters.
· The 'setplot delete' option has been enhanced to allow removal of all or a range of commands.
· For external programs calling XSPEC, new wrapper functions have been added for retrieving XFLT keywords from data files.
· Norm parameters are now set with a default 'soft' upper limit below their 'hard' upper limit.
· In PyXspec, the Fit.statMethod and statTest attributes can now be set for individual spectra.
· AtomDB has been upgraded to version 2.0.2.
· The tclout 'stat' and 'statmethod' options can now retrieve the test statistic as well as the fit statistic.
· The simftest Tcl script command now takes an optional filename argument for output.
· Attributes added to PyXspec classes: Xset.parallel, Fit.statTest.
All bug fixes to v12.8.0 released as patches are included in v12.8.1. In addition the following problems have been corrected:
· The command history file xspec.hty (in the user's ~/.xspec directory) is now updated when exiting XSPEC with the 'quit' command. Previously it was only updated when exiting with 'exit'.
· The 'chain' command can now read/write files in ASCII format when running in the default Goodman-Weare mode. Previously this feature was only available for Metropolis-Hastings chains.
· Fix to an array access error in the nthcomp model.
· PyXspec fix removes error messages generated when accessing response parameters in Python versions 2.6.x.
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
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
and additional models which can be added at
Further useful information can be found on the XSPEC Wiki at
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