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            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 in v12.8.2

New features:

·         The code underlying the NEI models has been rewritten to accommodate the new AtomDB 3.0 files, the beta version of which are included as the default when using any of the NEI models.  Older versions can be recovered using xset NEIVERS. The CEI models (apec etc) use some of the same code but there should be no change in their behavior except for a slight speed-up.

·               agauss – Gaussian model with units in Angstrom and a Gaussian shape in wavelength space.

·               nsx – Neutron star non-magnetic atmosphere model from Wynn Ho and collaborators.

·               nsmaxg – Neutron star magnetic atmosphere model.

·               rnei – NEI recombination model assuming an initial temperature.

·               vrnei – rnei with variable abundances for the common elements.

·               vvrnei – rnei with variable abundances for all elements (Z <= 30).

·               zagauss – The agauss model with a redshift parameter.

·      The goodness command has added options fit|nofit. The fit option runs a fit on each simulation before calculating the test statistic.

·      The cstat fit statistic can be modified by appending an integer eg. statistic cstat5. In this case the statistic will be evaluated as if the group min N had been used in grppha. This is not as efficient because the rebinning has to be done every statistic evaluation however it is usual for testing using simulations.

·      The ‘model’ command now checks that parameters are all within a reasonable order of magnitude (~1e10) of each other, and issues a warning if they are not.

·      Improved the handling of new-best-fit cases when running the ‘error’ command on parallel processes.  The global new-best-fit is now automatically retained.  Previously the user was prompted to re-run ‘error’ in a single process.

·      Models can now store (keyword,value) pairs using loadDbValue (C++)or PDBVAL (Fortran). These values can be recovered using the tclout(r) modkeyval option. This provides a way to get internal information from the last invocation of a model.

·      There is a manual XSFunctions_guide.pdf in Xspec/src/help describing some of the C++ classes used in the XSFunctions library. This is intended to make it easier to use these classes in other models.

Enhancements previously released as patches to 12.8.1:

·      PyXspec: added Fit.testStatistic attribute for retrieving the test statistic value from the most recent fit.

·      PyXspec: added compiler macros for switching to <Python/Python.h> include paths when building on Mac platforms.

All bug fixes to v12.8.1 released as patches are included in v12.8.2.  In addition the following problems have been corrected:

·      Fix for a particular case of non-associative expression evaluation.  Models of the form C((M1*A)M2) or C(M1*A)M2, where C is a convolution component, were being wrongly evaluated as (C(M1*A))M2.

·      For users with external programs linking to the XSPEC model library, the FNINIT function now properly initializes the AtomDB version to v2.0.2.

·      Fix for an ‘ignore’ command syntax evaluation that was causing a crash on OS X Mavericks.


How to find out more information

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.


Arnaud, K.A., 1996, Astronomical Data Analysis Software and Systems V, eds.  G. Jacoby and J. Barnes,p17, ASP Conf. Series volume 101.

Dorman, B., and Arnaud, K. A. 2001, Astronomical Data Analysis Software and Systems X eds.  F.R. Harnden, Jr., F.A. Primini, and H. E. Payne, vol. 238, p. 415

Dorman, B., Arnaud, K. A., and Gordon, C. A. XSPEC12: Object-Oriented X-Ray Data Analysis, AAS HEAD meeting No. 35, #22.10


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Next: Spectral Fitting and XSPEC Up: Manual Home Page Previous: Updates to the Manual