AN UPDATED CALIBRATION OF THE ROSAT PSPC PARTICLE BACKGROUND FOR THE ANALYSIS OF DIFFUSE AND EXTENDED SOURCES

In order to permit quantitative studies of the cosmic diffuse X-ray background (DXRB) and of extended X-ray sources, we present updated calibrations of the particle-induced background of the Position Sensitive Proportional Counters (PSPCs) on board the Rontgen Satellite (ROSAT). We present new parameterizations of the temporal, spectral, and spatial distributions of the particle-induced events following closely the analysis discussed in Snowden et al. (1992). The ROSAT Guest Observer (GO) may find a step-by-step method for applying these parameterizations to a GO observation in sectional sign 3.4. Except for a variable contamination which is present in channels less-than-or-equal-to 18 and a change in our understanding of the externally produced components, the current parameterizations are quite similar to the previous results. We have used the spectral information available on the variable contamination to formulate a method for determining the level of this contamination in a given observation. The PSPC rejection efficiency for particle background events in the pulse-height range 18 less-than-or-equal-to CH less-than-or-equal-to 249 is 99.90%, with a typical count rate of 4 x 10(-6) counts s-1 arcmin-2 keV-1. During typical conditions, the count rate of residual events is well correlated with the Master Veto count rate. The spectrum in the pulse-height range 18 less-than-or-equal-to CH less-than-or-equal-to 249 is well described by a power law, a flat component, and an Al Kalpha line at 1.5 keV. The spatial distribution of counts with pulse heights greater-than-or-equal-to 18 is uniform over the field of view except for a small radial gradient and shadowing of the Al Kalpha line and part of the flat continuum by the window support structure. During an astronomical observation in low-gain mode (after 1991 October 11), the particle background can also be monitored by the count rate in channels 260 less-than-or-equal-to CH less-than-or-equal-to 370, since in most cases all these events are produced by particles. We have used a 54 ks observation of the Ursa Major region to verify the accuracy of our model. We have also presented a summary of the particle background parameterizations valid for four different time epochs spanning the entire ROSAT mission, including a recalibration of the data presented previously.