# Simulating ACIS Pileup with marx¶

Confusion is a word we have invented for an order which is not understood.

– Henry Miller

## Introduction¶

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.
The 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.