Model Issues - II

The mdefine command.

  • We have implemented a new command to define a model component using an algebraic expression. This model component is evaluated by an interpreter so is slower than if it had been compiled. mdefine supports all standard arithmetic and trigonometric operations.

  • For example
    XSPEC> mdefine dplaw E**p1 + f*E**p2
    defines a model comprising the sum of two power-laws. Very sophisticated models are possible.
    XSPEC> mdef sg exp(-E^2/(2*A*E))/sqrt(6.283*A*sqrt(E)) : con
    defines a Gaussian convolution model whose sigma varies as the square root of energy.

Line identification.

  • One of the central tasks with high resolution spectral data will be to attempt to identify lines. We have added a new command to print out all possible lines in an energy range supplied by the user.

  • We have also written a Tcl script to automate line fitting and identification. This requires a continuum model to have been defined. The command adds successive lines (either gaussian or lorentzian) up to a number specified by the user. Initially it sets each line center at the energy with the largest residual between data and model then fits for the energy, width, and normalization of the line.