Simulating Data

The sim (with an alias called ``fakeit'' for xspec afficianados) will generate a simulated image corresponding to the input model, times a given scale value. In the case of a simulation based on a model fit to real data, the scale parameter sets the exposure of the simulated image in terms of the exposure of the input image (i.e., scale = 2 corresponds to twice the exposure of the input image). ximgfit uses this approach since the typical image will be in units of counts and may or may not have the exposure keyword(s) set properly. The simulated pixel values are drawn from a Poissian deviate with a mean value equal to the current model image pixel intensity times the scale value, and accordingly include photon counting statistics.