Adam Ingram and Chris Done note (appendix A of http://arxiv.org/abs/1108.0789) the following method of fitting models to power spectra generated by powspec. The output of powspec can be written to a text file with four columns f, df, P, dP where f is the frequency and P the power spectrum. This should be converted to a text file with the four columns being f-df, f+df, 2*P*df, 2*dP*df. This file can be used as the input to flx2xsp to create a "spectrum" file and diagonal response to read into xspec. Plots in xspec of flux vs. energy are then actually of power vs. frequency (eg. ufspec will give frequency multiplied by power vs. frequency).