Modify a response file gain, in a particularly simple way. 
CAUTION This
command is to be used with extreme care for investigation of the response
properties. To properly fit data, the response matrix should be recalculated
explicitly (outside of XSPEC) using any modified gain information derived.
XSPEC> gain [ fit | nofit | < <data set #> <slope> <intercept> >...
The initial default <data set #> is 1; later, the default is the number of the file last modified. Initially, all responses are assumed to have nominal gains, determined implicitly by the data in the response files. This is equivalent to a <slope> of 1 and an <intercept> of zero. If the fit argument is given then the <slope> and <intercept> for each dataset will be added to the model parameters and can be fit for. The nofit argument switches off the fitting and leaves the gain at the current values of the parameters.
Unless gain fit is in use the gain is automatically reset to nominal whenever a new data file or response file is defined for that file number, but not by any use of the ignore, notice, or dummyrsp commands.