JDPileup

class sherpa.astro.models.JDPileup(name='jdpileup')

Bases: sherpa.models.model.ArithmeticModel

A CCD pileup model for the ACIS detectors on Chandra.

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.

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/

Attributes Summary

thawedparhardmaxes
thawedparhardmins
thawedparmaxes
thawedparmins
thawedpars

Methods Summary

apply(outer, \*otherargs, \*\*otherkwargs)
calc(p, arf_source, exposure_time, ...)
get_center()
guess(dep, \*args, \*\*kwargs)	Set an initial guess for the parameter values.
reset()
set_center(\*args, \*\*kwargs)
startup()
teardown()

Attributes Documentation

thawedparhardmaxes

thawedparhardmins

thawedparmaxes

thawedparmins

thawedpars

Methods Documentation

apply(outer, *otherargs, **otherkwargs)

calc(p, arf_source, exposure_time, min_energy, max_energy, specresp, model)

get_center()

guess(dep, *args, **kwargs)

Set an initial guess for the parameter values.

Attempt to set the parameter values, and ranges, for the model to match the data values. This is intended as a rough guess, so it is expected that the model is only evaluated a small number of times, if at all.

reset()

set_center(*args, **kwargs)

startup()

teardown()