Simulating ACIS Pileup with marx¶
Confusion is a word we have invented for an order which is not understood.
– Henry Miller
In this section, we will discuss how to use the
marxpileup tool in
marx to simulate the effects on Chandra data of photon pileup in the
ACIS CCDs. As a post-processing module, the
marxpileup tool is designed to
work with existing marx simulations. Users first create a simulation
using marx Running marx simulations and then, if necessary, run
pileup on the results of that simulation to study pileup effects. In
this manner, users simulating faint or extended sources which are less
susceptible to pileup can produce marx simulations more quickly. We
will briefly describe the pileup process itself and then proceed to
outline the use and output of the pileup tool.
What is ACIS Pileup?¶
At some level, all Chandra observations performed with the ACIS imager will suffer from the effects of pileup. Pileup occurs when two or more photons land in the same pixel location in a given ACIS readout time. In this situation, ACIS “detects” a single photon with an energy which is roughly the sum of the two component photons. Some simple schematic representations of such “piled” photons are shown below. The pileup process can affect ACIS data in a number of ways including:
- Photometric Inaccuracy
The event detection algorithm cannot distinguish between single, large pulse height events and composite, piled events. Consequently, the detected count rate will be reduced with respect to the true count rate in the absence of pileup.
- Spectral Distortion
By combining multiple incident, photons into a single “detected” event with a larger pulse height, pileup effectively “hardens” the observed ACIS spectrum.
- Point Spread Function Distortion
The severity of the pileup effect is governed in part by event density. Since the core of the PSF has a higher event density than the wings, it will be affected to a greater degree. This effect will tend to broaden the PSF as the ratio of core to wing events decreases.
- Grade Migration
As the degree of pileup increases, the distribution of event grades will change. Multiple photon or “piled” events will tend to have “bad” grades which include detached corner pixels, such as ASCA grades 1,5, and 7. This migration will have repercussions for standard data analysis which often begins by discarding such “bad” grades.
Chandra observations of bright point sources are the most likely to be significantly affected by pileup. Extended or faint sources will be less affected although sharp features such as unresolved cores or bright knots or filaments could still be vulnerable. Although pileup cannot be avoided entirely, a number of techniques can be employed to mitigate its effects somewhat. For a more detailed discussion of pileup, the reader is referred to acis_pileup.
Overview of the pileup Tool¶
marxpileup tool implements the pileup algorithm developed by John Davis
(MIT). This same algorithm has been implemented into the ISIS,
SHERPA, and XSPEC spectral fitting packages. While this implementation
of the pileup algorithm emulates most of the qualitative effects of ACIS
photon pileup, users should keep in mind that we are still calibrating
the procedure. The ACIS pileup model is statistical and is not an a
priori photon-silicon interaction model which generates charge clouds
and then PHAs per event “island.” The model is valid on-axis for point
sources for low to moderate pileup. While valid for qualitative
predictions of the effects of pileup on the PSF, it has not been
verified for image reconstruction. Detailed studies of the effects of
pileup on the HRMA PSF including comparisons to actual on-orbit data are
still underway. The model is very good for spectral modeling of light to
moderately piled point sources. Users should interpret all results
including the effects of pileup cautiously.