Synopsis
A CCD pileup model for the ACIS detectors on Chandra.
Syntax
jdpileup
Description
This model is based on the work by John Davis ( [1] ). It is intended only for modeling the pileup in one-dimensional spectra obtained in imaging mode (i.e. no grating data), but can be used with the zeroth-order spectrum of a grating data set.
Example
>>> create_model_component("jdpileup", "mdl")
>>> print(mdl)Create a component of the jdpileup model and display its default parameters. The output is:
mdl Param Type Value Min Max Units ----- ---- ----- --- --- ----- mdl.alpha thawed 0.5 0 1 mdl.g0 frozen 1 1.17549e-38 1 mdl.f thawed 0.95 0.9 1 mdl.n frozen 1 1.17549e-38 100 mdl.ftime frozen 3.241 1.17549e-38 5 sec mdl.fracexp frozen 0.987 0 1 mdl.nterms frozen 30 1 100
ATTRIBUTES
The attributes for this object are:
| Attribute | Definition |
|---|---|
| alpha | The alpha parameter parameterizes "grade migration" in the detector and represents the fraction of piled-up events that result in a good grade. |
| g0 | The probabilty of assigning a grade of zero. This should remain frozen. |
| f | The fraction of flux falling into the pileup region. This should remain frozen. |
| n | The number of detection cells. This parameter can not be fit. |
| ftime | The frame time in seconds (as given by the exptime keyword of the event file). This parameter can not be fit. |
| fracexp | The fractional exposure that the source experienced while dithering on the chip (as given by the fracexpo keyword in the ARF file). This parameter can not be fit. |
| nterms | The maximum number of photons considered for pileup in a single readout frame. This should not be changed from its default value of 30. This parameter can not be fit. |
Notes
As the pileup model is inherently non-linear, it is strongly advised that multiple optimization methods are used to thoroughly investigate the search space for the model.
The alpha parameter should vary with photon energy and detector position, but for simplicity it is treated as independent of energy and location.
An example of using this model to fit a Chandra spectrum is provided in [2] .
References
- [1] Davis, J, 2001, ApJ, 562, 575-582. http://adsabs.harvard.edu/abs/2001ApJ...562..575D
- [2] "Using A Pileup Model", http://cxc.harvard.edu/sherpa/threads/pileup/
Bugs
See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.