Filtering of ACIS Events

The effective area of the HRMA/ACIS combination at a given energy does not have a unique value but in general is a function of several parameters, the values of which are selected based on the scientific goals of a particular ACIS observation and the nature of the observed spectrum. As an example, in some situations an observer may wish to sacrifice energy resolution for increased effective area by accepting a larger number of ACIS grades. We therefore have determined the effective area and energy resolution (FWHM) of ACIS as a function of split event threshold, CCD grade, and off-axis angles.

For the reduction of ACIS data acquired at XRCF we have made use of the interactive tool named Event Browser (EB) and an automated event filtering tool named AcisFilter based on EB. The interactive tool is used to initially inspect each event file to be processed and to determine the spectral ROI's, the good time intervals and the centroid of the focussed beam on the CCD. The automated event filtering tool is used to filter ACIS events by grade, time, split event thresholds, and circular or rectangular spatial regions for a given amplifier of a given CCD. It also computes the CCD ontime, t_live, for filtered events.

To reduce the data processing time and to allow the addition of possible future enhancements without having to manually repeat all the steps we have incorporated the various input parameters (ie. ACIS event filenames, split event thresholds, grade selections, ROI's in spectral, spatial and time domain) in parameter files that are read in by the event filtering software tool.