THE ENERGY-DEPENDENT TEMPORAL VARIATION OF THE ROSAT PSPC GAIN

The existence of systematic spectral-fit residuals in ROSAT PSPC spect ra, and their dependence on time, is by now a well established fact. T his paper describes how those residuals may be related to second order variations of the gain of the PSPC as a function of energy and time. As a result, the energy scale used for the interpretation of PSPC spec tra can be incorrect producing significant systematic effects, in part icular at energies where the effective area of the instrument changes rapidly. A monotonic gain decay of similar to 1% per year is measured at 1 keV. A quantitative description of the PSPC gain-time variation a s a function of energy is provided. The functional dependence of this variation is found to be well represented by a second order polynomial .