Due to the lapse in federal government funding, NASA is not updating this website. We sincerely regret this inconvenience.
An X-Ray Spectral Fitting Package
Users' Guide for version 12.15.1
Keith Arnaud, Craig Gordon, Ben Dorman & Kristin Rutkowski
HEASARC
Astrophysics Science Division
NASA/GSFC
Greenbelt, MD 20771
September 2025
- Contents
- Introduction
- Spectral Fitting and XSPEC
- Introduction
- The Basics of Spectral Fitting
- The XSPEC implementation
- Fits and Confidence Intervals
- A more abstract and generalized approach
- XSPEC Data Formats
- References
- XSPEC Overview and Helpful Hints
- Syntax
- How to return to the XSPEC12>prompt
- Getting Help
- Commands
- Issuing Commands
- Control Commands
- Query, chatter and shutting XSPEC up (somewhat)
- Scripts and the Save command
- Miscellaneous
- Data Commands
- Model Commands
- Fitting Commands
- Binning and Grouping data
- Plotting Commands
- Setting Commands
- Breaking With Ctrl-C
- Customizing XSPEC
- Customizing system-wide
- Walks through XSPEC
- Introduction
- Brief Discussion of XSPEC Files
- Fitting Models to Data: An Old Example from EXOSAT
- Simultaneous Fitting
- Multiple Models: a Background Modeling Example
- Using XSPEC to Simulate Data: an Example for Chandra
- Producing Plots: Modifying the Defaults
- Markov Chain Monte Carlo Example
- Spectropolarimetry Example
- XSPEC Commands
- Summary of Commands
- Description of Syntax
- Control Commands
- Data Commands
- Fit Commands
- Model Commands
- Plot Commands
- Setting Commands
- Tcl Scripts
- XSPEC Models
- Alphabetical Summary of Models
- Additive Model Components
- agauss, zagauss: gaussian line profile in wavelength space
- agnsed, qsosed: AGN SED model
- agnslim, AGN super-Eddington accretion model
- apec, vapec, vvapec, bapec, bvapec, bvvapec: APEC emission spectrum
- atable: tabulated additive model
- bbody, zbbody: blackbody
- bbodyrad: blackbody spectrum, area normalized
- bexrav: reflected e-folded broken power law, neutral medium
- bexriv: reflected e-folded broken power law, ionized medium
- bkn2pow: broken power law, 2 break energies
- bknpower, zbknpower: broken power law
- bmc: Comptonization by relativistic matter
- bremss, vbremss, zbremss: thermal bremsstrahlung
- btapec, bvtapec, bvvtapec: velocity broadened APEC emission spectrum with separate continuum and line temperatures
- bwcycl: Becker-Wolff self-consistent cyclotron line model
- c6mekl, c6vmekl, c6pmekl, c6pvmkl: differential emission measure using Chebyshev representations with multi-temperature mekal
- carbatm: Nonmagnetic carbon atmosphere of a neutron star
- cemekl, cevmkl: plasma emission, multi-temperature using mekal
- cempow, vcempow, bcempow, bvcempow: plasma emission, multi-temperature using a power-law emission measure
- cflow: cooling flow
- cheb6, vcheb6, bcheb6, bvcheb6: differential emission measure using 6-order Chebyshev polynomial
- cie, vcie, vvcie, bcie, bvcie, bbvcie: Emission spectrum from a plasma in Collisional-ionization equilibrium
- compbb: Comptonization, black body
- compLS: Comptonization, Lamb & Sanford
- compmag: Thermal and bulk Comptonization for cylindrical accretion onto the polar cap of a magnetized neutron star
- compPS: Comptonization, Poutanen & Svensson
- compST: Comptonization, Sunyaev & Titarchuk
- comptb: Thermal and bulk Comptonization of a seed blackbody-like spectrum
- compTT: Comptonization, Titarchuk
- coolflow, vcoolflow, bcoolflow, bvcoolflow: cooling flow, mekal
- cph, vcph, bcph, bvcph: Cooling + heating model for cool core clusters
- cplinear: a non-physical piecewise-linear model for low count background spectra.
- cutoffpl, zcutoffpl: power law, high energy exponential cutoff
- disk: accretion disk, black body
- diskbb: accretion disk, multi-black body components
- diskir: Irradiated inner and outer disk
- diskline: accretion disk line emission, relativistic
- diskm: accretion disk with gas pressure viscosity
- disko: accretion disk, inner, radiation pressure viscosity
- diskpbb: accretion disk, power-law dependence for T(r)
- diskpn: accretion disk, black hole, black body
- eebremss: electron-electron bremsstrahlung spectrum
- eplogpar: log-parabolic blazar model with
F
normalization
- Eqpair, eqtherm, compth: Paolo Coppi's hybrid (thermal/non-thermal) hot plasma emission models.
- equil, vequil, bequil, bvequil: collisional plasma, ionization equilibrium
- expcheb6, vexpcheb6, bexpcheb6, bvexpcheb6: differential emission measure using exponential of a 6-order Chebyshev polynomial
- expdec: exponential decay
- ezdiskbb: multiple blackbody disk model with zero-torque inner boundary
- fekblor, zfekblor, bfekblor, zbfekblor: Fe K
line at high resolution
- feklor, zfeklor, bfeklor, zbfeklor: Fe K
line at high resolution
- gadem, vgadem, vvgadem, bgadem, bvgadem, bvvgadem: plasma emission, multi-temperature with gaussian distribution of emission measure.
- gauss, zgauss: gaussian line profile
- gnei, vgnei, vvgnei, bgnei, bvgnei, bvvgnei: collisional plasma, non-equilibrium, temperature evolution
- grad: accretion disk, Schwarzschild black hole
- grbcomp: Comptonization for GRB prompt emission
- grbjet: Two-phase Comptonization model of soft thermal seed photons for GRB prompt emission
- grbm: gamma-ray burst continuum
- hatm: Nonmagnetic hydrogen atmosphere of a neutron star
- jet: Leptonic relativistic jet model
- kerrbb, zkerrbb: multi-temperature blackbody model for thin accretion disk around a Kerr black hole
- kerrd: optically thick accretion disk around a Kerr black hole
- kerrdisk: accretion disk line emission with BH spin as free parameter
- kyrline: relativistic line from axisymmetric accretion disk
- laor: accretion disk, black hole emission line
- laor2: accretion disk with broken-power law emissivity profile, black hole emission line
- logpar, zlogpar: log-parabolic blazar model
- lorentz, zlorentz: lorentz line profile
- meka, vmeka: emission, hot diffuse gas (Mewe-Gronenschild)
- mekal, vmekal: emission, hot diffuse gas (Mewe-Gronenschild-Kaastra)
- mkcflow, vmcflow: cooling flow, mekal
- nei, vnei, vvnei, bnei, bvnei, bvvnei: collisional plasma, non-equilibrium, constant temperature
- nlapec: continuum-only APEC emission spectrum
- npshock, vnpshock, vvnpshock, bnpshock, bvnpshock, bvvnpshock: shocked plasma, plane parallel, separate ion, electron temperatures
- nsa: neutron star atmosphere
- nsagrav: NS H atmosphere model for different g
- nsatmos: NS Hydrogen Atmosphere model with electron conduction and self-irradiation
- nsmax: Neutron Star Magnetic Atmosphere
- nsmaxg: Neutron Star with a Magnetic Atmosphere
- nsx: neutron star with a non-magnetic atmosphere
- nteea: non-thermal pair plasma
- Nthcomp: Thermally comptonized continuum
- Optxagnf, optxagn: Colour temperature corrected disc and energetically coupled Comptonisation model for AGN.
- pegpwrlw: power law, pegged normalization
- pexmon: neutral Compton reflection with self-consistent Fe and Ni lines.
- pexrav: reflected powerlaw, neutral medium
- pexriv: reflected powerlaw, ionized medium
- plcabs: powerlaw observed through dense, cold matter
- polconst: Constant polarization
- pollin: Linearly dependent polarization
- polpow: Power-law dependent polarization
- posm: positronium continuum
- powerlaw, zpowerlw: power law photon spectrum
- pshock, vpshock, vvpshock, bpshock, bvpshock, bvvpshock: plane-parallel shocked plasma, constant temperature
- raymond, vraymond: emission, hot diffuse gas, Raymond-Smith
- redge: emission, recombination edge
- refsch: reflected power law from ionized accretion disk
- relline, relline_lp, relline_lp_ext: relativistic accretion disk line emission with different geometries
- rnei, vrnei, vvrnei, brnei, bvrnei, bvvrnei: non-equilibrium recombining collisional plasma
- rsapec, rsvapec, rsvvapec: APEC emission spectrum with resonance scattering
- rsgauss: gaussian line profile with resonance scattering
- rsrnei, rsvrnei, rsvvrnei: Non-equilibrium recombining collisional plasma with resonance scattering
- sedov, vsedov, vvsedov, bsedov, bvsedov, bvvsedov: sedov model, separate ion, electron temperatures
- sirf: self-irradiated funnel
- slimbh: Stationary slim accretion disk
- smaug: optically-thin, spherically-symmetric thermal plasma.
- snapec, bsnapec: galaxy cluster spectrum using SN yields
- srcut: synchrotron spectrum, cutoff power law
- sresc: synchrotron spectrum, cut off by particle escape
- ssa: Strangeon star atmosphere
- sssed: Shakura & Sunyaev spectral energy distribution
- step: step function convolved with gaussian
- tapec, vtapec, vvtapec: APEC emission spectrum with separate continuum and line temperatures
- wdem, vwdem, vvwdem, bwdem, bwwdem, bvvwdem: plasma emission, multi-temperature with power-law distribution of emission measure.
- vagauss, zvagauss: gaussian line profile in wavelength space with sigma in velocity
- vgauss, zvgauss: gaussian line profile with sigma in velocity
- vlorentz, zvlorentz: lorentz line profile with width in velocity
- voigt, zvoigt: Voigt line profile
- vvoigt, zvvoigt: Voigt line profile with widths in km/s
- Multiplicative Model Components
- absori: ionized absorber
- acisabs: Chandra ACIS q.e. decay
- cabs: Optically-thin Compton scattering.
- constant: energy-independent factor
- cyclabs: absorption line, cyclotron
- dust: dust scattering
- edge, zedge: absorption edge
- etable: exponential tabular model
- expabs: exponential roll-off at low E
- expfac: exponential modification
- gabs, zgabs: gaussian absorption line
- heilin: Voigt absorption profiles for He I series
- highecut, zhighect: high-energy cutoff
- hrefl: reflection model
- ismabs: A high resolution ISM absorption model with variable columns for individual ions
- ismdust: Extinction due to a power-law distribution of dust grains
- logconst: Constant in log units
- log10con: Constant in base 10 log units
- lorabs, zlorabs: lorentzian absorption line
- lyman: Voigt absorption profiles for H I or He II Lyman series
- mtable: multiplicative tabular model
- notch: absorption line, notch
- olivineabs: Absorption due to olivine.
- pcfabs, zpcfabs: partial covering fraction absorption
- phabs, vphabs, zphabs, zvphabs: photoelectric absorption
- plabs: power law absorption
- pwab: power-law distribution of neutral absorbers
- redden: interstellar extinction
- smedge: smeared edge
- spexpcut: super-exponential cutoff absorption
- spline: spline modification
- SSSice: Einstein SSS ice absorption
- swind1: absorption by partially ionized material with large velocity shear
- tbabs, ztbabs, tbfeo, tbgas, tbgrain, tbpcf, tbvarabs, tbrel: ISM grain absorption
- uvred: interstellar extinction, Seaton Law
- varabs, zvarabs: photoelectric absorption
- vgabs, zvgabs: gaussian absorption line with sigma in km/s
- vlorabs, zvlorabs: lorentzian absorption line with width in km/s
- voigtabs, zvoigtabs: voigt absorption line
- vvoigtabs, zvvoigtabs: voigt absorption line with widths in km/s
- wabs, zwabs: photoelectric absorption, Wisconsin cross-sections
- wndabs, zwndabs: photo-electric absorption, warm absorber
- xion: reflected spectrum of photo-ionized accretion disk/ring
- xscat: dust scattering
- zbabs: EUV ISM attenuation
- zdust: extinction by dust grains
- zigm: UV/Optical attenuation by the intergalactic medium
- zredden: redshifted version of redden
- zsmdust: extinction by dust grains in starburst galaxies
- zvfeabs: photoelectric absorption with free Fe edge energy
- zxipab: power-law distribution of ionized absorbers
- zxipcf: partial covering absorption by partially ionized material
- Convolution Model Components
- cflux: calculate flux
- cglumin: calculate luminosity
- clumin: calculate luminosity
- cpflux: calculate photon flux
- gsmooth: gaussian smoothing
- ireflect: reflection from ionized material
- kdblur: convolve with the laor model shape
- kdblur2: convolve with the laor2 model shape
- kerrconv: accretion disk line shape with BH spin as free parameter
- kyconv: convolution using a relativistic line from axisymmetric accretion disk
- lsmooth: lorentzian smoothing
- partcov: partial covering
- rdblur: convolve with the diskline model shape
- reflect: reflection from neutral material
- relconv, relconv_lp, relconv_lp_ext: convolve with relativistic accretion disk line profile with different geometries
- rfxconv: angle-dependent reflection from an ionized disk
- rgsext: convolve an RGS spectrum for extended emission
- rgsxsrc: convolve an RGS spectrum for extended emission
- simpl: comptonization of a seed spectrum
- ThComp: Thermally comptonized continuum
- vashift: velocity shift an additive model
- vmshift: velocity shift a multiplicative model
- xilconv: angle-dependent reflection from an ionized disk
- zashift: redshift an additive model
- zmshift: redshift a multiplicative model
- Pile-Up Model Components
- Mixing Model Components
- ascac: ASCA surface brightness model
- clmass, nfwmass, monomass: Cluster mass mixing models
- crossarf: mixing model for psf corrections when fitting multiple regions
- projct: project 3-D ellipsoidal shells onto 2-D elliptical annuli
- recorn: change correction norm for a spectrum
- polrot: Rotate polarization
- suzpsf: suzaku surface brightness model
- xmmpsf: xmm surface brightness model
- Appendix A: The User Interface
- Appendix B: Statistics in XSPEC
- Appendix C: Adding Models to XSPEC
- Appendix D: Overview of PLT
- Appendix E: Associated Programs
- Appendix F: Using the XSPEC Models Library in Other Programs
- Appendix G: Adding a Custom Chain Proposal Algorithm
- Appendix H: Changes between v11 and v12
- Appendix I: Older Release Notes
- Appendix J: Glossary
- About this document ...