recorn: change correction norm for a spectrum

This model is a replacement for and improvement on the old xspec command recornrm. If a correction file is in use for a spectrum then its normalization can be fitted for using this model. The first parameter, which is not variable, is the spectrum number and the second the correction file normalization. The starting value of the second parameter should be set to the current value of the correction file norm (this can be independently set using the cornorm command).

Note that in order to fit the cornorm parameter the calculation of parameter derivatives during the fit must be switched to the slower full numerical differentiation algorithm. This can be done either by

XSPEC12>xset usechainrule no

or by changing the USE_CHAIN_RULE setting in the Xspec.init start-up file to false. This does slow down the fit so is not recommended for general use.

par1	specnum: spectrum number
par2	cornorm: correction file normalization

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Last modified: Wednesday, 12-Mar-2025 17:05:25 EDT