ACIS pileup occurs in the HETGS dispersed spectra as in all ACIS observations. For a point source the dispersed events are in narrow linear spectra and an estimate of pileup effects along a spectrum is given by the event rate per pixel island; that is by the rate:
number of events in 3 pixel bin along the dispersion axis Island Rate = -------------------------------------------------------- number of ACIS readout framesThe value of this rate indicates the magnitude of pileup effects: for values of 0.01 and less pileup effects are negligible; for values of 0.01 to 0.1 pileup will produce changes in event rates less than 10%; for values much above 0.1 pileup can be severe and the data quality dubious.
Pileup plots indicating this rate are presented for several calibration sources and show the varying levels of pileup in the HETGS data sets. Similar plots can be made from simulated data sets to check the expected pileup level for an observation.
Pileup Plots for several calibration targets are available below. The plots were made with HAK code version 1.3 with this updated version of obs_anal.pro.
The Y axis shows the number of events summed over 3 pixels - specifically, "AX" is binned to 1 pixel bins and then convolved with a box-car kernal, [1.,1.,1.]. The resulting histogram gives the numerator of the island rate as a function of the AX (~ TDETX) value. Note that for simplicity all events in the cross-dispersion are included as the bulk of events are concentrated within a pixel or two of the dispersion axis. No ACIS ENERGY selction is applied so that all orders are included.
The number of ACIS readout frames, the denominator of the island rate, is shown in the plots by the solid red line. Thus this level corresponds to an island rate of 1 event per frame; dashed and dotted red lines indicate island rates of 0.1 and 0.01 per frame. The number of frames used here is the number of frames in the input data set for all CCDs - in the high event rate case (e.g., Cyg X-2 below) the actual number of frames is a function of the CCD and can introduce a factor of 2 error as the red line is approached.
- Mkn 421, Obsid 457
- Capella, Obsid 1318
- Crab Pulsar and Nebula, Obsid 168
- Cyg X-2, Obsid 1102
Please send any comments to
Dan Dewey at email@example.com.