|HEAD 2002 Pileup Talk|
Data Preparation and Caveats
Before one can actually use the pileup model, one must first filter the data appropriately and then construct PHA files, RMFs, and ARFs.
Do not use level 2 files!
The most important thing to note is that you must start with level 1 files and not with level 2 files. The main reason for this is that level 2 files have had so-called afterglow events removed. For bright sources this means that 10 to 20 percent of the events from the source have been misidentified as cosmic ray events and removed from the level 2 files. Hence, you have to go back to the level 1 files. See the CIAO acis_detect_afterglow thread for information about how to keep afterglow events.
Exercise care in creating light curves
The pileup model assumes that photon arrival times are Poisson distributed. This means that lightcurves must be constructed to determine regions where the incident source flux is fairly constant. In doing so, one might be tempted to use events centered upon the source itself for the construction of the lightcurves. However, this is a bad idea since for a bright source, the relationship between the source flux and the count-rate is non-linear. Instead, use either the events in the wings of the PSF, or the events in the ACIS readout streak. To illustate this point, here is a lightcurve of II Peg (obsid_1451) constructed from the source events:
Use the right extraction regionFor an on-axis point source, it is recommended that a circular region with a 2 arc-second radius be used (4 ACIS pixels). Such a region encompasses 95 percent of the incident flux and one would have to go out to a much larger radius to get the remaining 5 percent at the risk of decreasing the signal to noise ratio. Schematically, the recommended extraction looks like
Do not use a canned ARFUse
Get the EXPOSURE right!The pileup model uses the frame-time and exposure time to compute the number of CCD frames. If the value of the EXPOSURE keyword in the ARF or in the PHA file is wrong, then the number of frames will be wrong, causing the pileup model to fail. It has been our experience that a
This page was last updated May 31, 2002 by John E. Davis. To comment on it or the material presented here, send email to email@example.com.