Optimizing the ACIS effective area and energy resolution

The ACIS flight instrument was recently extensively calibrated at the X-ray Calibration Facility at MSFC. For the analysis of a subset of the ACIS calibration data we have employed an automated event filtering software package developed at PSU. We present results describing the dependence of the Effective Area and Energy Response of ACIS/HRMA as a function of grade selection, split event threshold, CCD and CCD amplifier, and off-axis angle. The main goal of this study is to facilitate the selection of the appropriate ACIS parameters that optimize a desired feature in an observed ACIS spectrum and may also guide observers in selecting the appropriate CCD for the observation. Optimizing a particular feature in an ACIS observation in general may require making a trade-off between Effective Area and Energy Response of AXAF/ACIS. The present analysis will facilitate the selection of the appropriate grades, CCD, CCD amplifier and split event thresholds needed to attain the optimal point. As an illustration of the effectiveness of this approach we present several case studies of typical astrophysical source spectra in which we enhance a particular scientific feature in the observed ACIS spectrum by the appropriate selection of ACIS parameters.