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HRI: Limitations

The study of extended sources with the HRI is more problematic than using the PSPC. The relative level of the noncosmic background is considerably higher and is dominated by the unvigneted particle background. In addition, less information is available to model the particle background, and much of what there is must be derived from data sets not specifically designed for that purpose.

The current software will only cast the events, cast a modeled particle background, and cast the effective exposure. The detector maps, both vignetted and particle background, are derived from the bright-Earth/dark-Earth data sets produced by SASS in the normal course of the data reduction. A selection on count rate is performed to isolate times when the satellite is viewing the Sun-lit (scattered solar X-rays provide a vignetted detector map) and dark Earth (the observed events are primarily particle background). In addition, a scaling and subtraction of the bright-Earth map from the dark-Earth map is performed to remove the effect of residual vignetted X-rays. A selection on zenith angle ( ) is also done.

The modeling of the particle background is done by using the average Master Veto count rate of the PSPC observed as a function of geographic subsatellite position. This quantity is smoothly varying and provides a reasonable monitor of the average particle-background rate. In order to convert it into a monitor of the HRI background, ROSAT TREND data (a data set containing monitor count rates in 60-second intervals produced on a daily basis) are used to create a scatter plot of HRI primary count rate versus PSPC Master Veto count rate determined by geographic position. The lower envelope of the scatter plot provides the relation between Master Veto and HRI particle background, plus a contribution from the cosmic background plus any other noncosmic background components. It is assumed that on average the contribution of all other components is a constant 0.5 counts s . The relation between HRI particle background count rate and average Master Veto count rate varies over the mission and is calibrated using data in  month time intervals when both the HRI and PSPC were operational, one-month intervals after the exhaustion of the PSPC counter gas.