A comparison of spectral reduction techniques in X-ray astronomy

The results of a numerical investigation of three methods of cosmic X-ray spectral reduction are reported, with an emphasis on the apodization technique of Dolan (1972). The study is carried out in the energy range 10-100 keV on pulse height distributions which were synthesized from simple X-ray source spectra with predetermined parameters. The spectral response function used was that of a balloon-borne, large area, xenon-filled proportional counter; however, the authors believe that the results are generally relevant to other non-dispersive systems such as scintillators and low energy (1-10 keV) proportional counters. They agree that the ill-conditioned energy resolution matrix cannot be inverted using standard procedures without propagating unacceptable errors. The apodization solution was investigated as a function of counter resolution, fluorescence escape probability, extent of energy window, number of pulse height channels, data S/N ratio, and the so-called apodization instrument factor. It is concluded that in most circumstances apodization should be the preferred reduction technique, but that the present widely and improperly used minimum -chi2 method may be useful as the final step in obtaining refined spectral parameter estimates. (Auth.)