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.
· Two of the remaining unimplemented v11 commands have now been added.
· mdefine allows dynamic definition of models that can be expressed algebraicly.
· recornrm has been replaced by the recorn model. This allows the correction norm to be treated as a fit parameter, a better solution than the v11 recornrm command.
· The complete HTML help files are included in a tar file. These can be made available on a local machine if remote access is now available and selected in the Xspec.init file.
· Convolution components can now operate on multiplicative components. For example, in the model = (CM)A, the convolution component acts on only the multiplicative component. Previously this would have been treated the same as C(MA). The partcov partial convolution model takes advantage of this new capability.
· There is a new simple way of estimating fluxes (and their errors) from parts of the model. Apply the cflux convolution model to the component(s) for which the flux is required.
· The following models have been added as standard
· diskir : irradiated disk
· kerrdisk : broad iron line from a disk around a Kerr BH
· nsmax : NS magnetic atmosphere
· nthcomp : thermally Comptonized continuum
· spexpcut : super-exponential cut-off
· swind1 : partially ionized absorbing material with velocity shear
· zxipcf : partial covering of partially ionized absorbing material
· cflux : calculate the flux from model component(s)
· kerrconv : broadening due to rotation around a Kerr BH
· partcov : partial covering modifier for absorption models
· simpl : Comptonization of a seed spectrum
· recorn : Vary the correction file normalization
· The lrt.tcl and simftest.tcl scripts perform the likelihood ratio and F-tests, respectively.
· The writefits.tcl script writes filenames and current fit parameters and errors to a single row of a FITS file. This script can be used as a template for saving other information.
· A response of "/*" to a "y/n" prompt will jump out of the current operation and return to the XSPEC command prompt. This is particularly useful for escaping nested fits during an error command run.
· The units have been changed for setplot wave plots. model and ufspec have a y-axis in photons/cm^2/s/Hz, emodel and eufspec in Jy (10^-23 erg/cm^2/s/Hz), eemodel and eeufspec in erg/cm^2/s.
· Fakeit can now work with multiple-extension response files. It also works correctly when multiple models are in use (this was release in patch v12.4.0r).
· The active|inactive|options can be applied to the default (unnamed) model (released in patch v12.4.0v).
· Support for GLAST GBM extensions to the standard file formats including multiple response matrix extensions in the same file (released in patch 12.4.0am).
· There are additional diagnostics available at high chatter levels from MCMC chain runs. User's custom proposal classes have access to information about acceptances and rejections.
· Initial support for multicore processors using the OpenMP parallel processing compiler option. Multiplication of the model and response is performed in parallel across the multiple spectra in a datagroup.
· All bug fixes to v12.4.0 released as patches a - ar are included in v12.5.0. In addition the following problems have been corrected.
· When a runtime error is encountered during the calculation of a parameter's error bounds (using the error command), the value is now filled in with 0.0 rather than retaining its previous value.
· Steppar will now correctly step in reverse direction if the range values were entered in high-to-low order.
· Model expression parsing has been improved for nested expressions.
· Log file output has been fixed so '#' comments are placed correctly.
· The chi-square calculation includes the corfile contribution even if there is no background file associated with the spectrum.
· There are minor plotting fixes to the confidence line in 1-D steppar/margin plots, the rescaling of the Y=0 green line in lower-panel plots, and the Y-axis label in plot delchi.
· Tclout peakrsid no longer fails for a spectrum whose model was not assigned to source 1.
· The XSFunctions library now also depends on XSModel, requiring the addition of a -lXSModel flag to the Makefile of external programs linking with the XSPEC model functions library. (See Appendix F)
· The modelIonData model data files directory has been renamed to modelData.
· Portions of some model functions have been translated from Fortran to C++ to reduce use of the udmget memory allocation function. Future versions will remove all references to udmget.
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