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Setting Commands

The fit and goodness-of-fit test statistics are set using the statistic command. Other fit-minimization algorithms are available, and can be selected using the method command. The various fit methods require first and in some cases second derivatives of the statistic with respect to the parameters. By default XSPEC calculates these analytically, using an approximation for the second derivatives. This may be changed by setting the USE_CHAIN_RULE flag in the user's startup Xspec.init file to false. The weighting algorithm used to calculate $\chi^2$ can be altered by the weight command.

Other setting commands modify:

  • cosmological parameters used to calculate luminosity (cosmo)
  • solar abundances for 18 elements (abund)
  • photoionization cross-sections (xsect)
The xset command can be used as an interface for abund, cosmo, method, statistic, and xsect. Additionally, xset may set string expressions that are used by models, for example the path to, and version number of AtomDB atomic calculations, or the coordinate system for surface brightness calculations used in the xmmpsf mixing model.