Temporal and Spatial Gain Corrections for the ROSAT PSPC

Abstract

A calibration and a method for correction of the residual temporally dependent spatial-gain variation of the ROSAT PSPC are presented. When combined with the correction for the Prieto, Hasinger, & Snowden temporally dependent energy nonlinearity, these corrections provide a significant improvement in the consistency of fitted spectral parameters over the life of the PSPC. To derive the correction for the spatial gain variation, we use Al Kα data (E ∼ 1.49 keV) from the on-board calibration sources to determine the gain as a function of detector position and time for 14, ∼100 day intervals throughout the calibration/verification and Guest Observer phases of ROSAT operations. The resulting gain maps show residual variations of up to ±4% in the gain around the mean channel used for the nominal PHA to PI channel conversion. The Prieto et al. results, an energy nonlinearity that peaks at ∼0.7 keV with a maximum reduction in the effective gain of ∼4.5%, and gain maps produced here are incorporated into correction algorithms that can be used on individual observations. We demonstrate the correction process using observations of the LMC supernova remnant N132D and the "Meaty" white dwarf performed at various times during the PSPC lifetime and at different positions across the detector for N132D. We find a significant improvement in the agreement of the fitted spectral parameter for the N132D spectrum, that has its peak emission at ∼0.8 keV. The Meaty results are marginal as the spectrum is particularly soft and not strongly affected by the Prieto et al. nonlinearity. The combined corrections aid in the comparison of source spectra from multiple observations spaced both in time and in position on the detector.