HEAD 2002 Pileup Talk  

NGC 4579

 

This example concerns the ACIS-S observation (obsid_807) of NGC 4579 by Michael Eracleous. After obtaining the best fit using isis fit_counts function, the list_par was used to get the fit_parameters:

isis> fit_counts;

 Parameters[Variable] = 7[5]
            Data bins = 82
           Chi-square = 103.6
   Reduced chi-square = 1.346

isis> list_par;

phabs(1)*powerlaw(1)
 idx  param             tie-to  freeze         value          min          max
  1  phabs(1).nH            0     0        0.02727911            0          0.2
  2  powerlaw(1).norm       0     0       0.001469524            0          0.1
  3  powerlaw(1).PhoIndex   0     0          1.764569            1            3
  4  pileup<1>.nregions     0     1                 1            1           10
  5  pileup<1>.g0           0     1                 1          0.8            1
  6  pileup<1>.alpha        0     0         0.6937202            0            1
  7  pileup<1>.psffrac      0     0          0.902147          0.8            1

The isis rplot_counts function was used to show how well the model reproduced the data:

Residual plot showing best fit vs data
The print_kernel function shows the degree of pileup:
isis> print_kernel(1);

 1: 0.367835     0.688744
 2: 0.186795     0.242635
 3: 0.0632391    0.0569848
 4: 0.0160571    0.0100375
 5: 0.00326167   0.00141443
 6: 0.000552117  0.000166095
 7: 8.01078e-05  1.67181e-05
 8: 1.01701e-05  1.47239e-06
 *** pileup fraction: 0.311256

This indicates that about 37 percent of the frames consisted of single photon events in the pileup regions, 19 percent were 2 photon events, etc. However, 69 percent of the observed events were single photon events, 24 percent were due to 2 photons, and so on. This gave a 31 percent pileup fraction.

The following figure, produced by isis using its confidence mapping and plotting functions, shows the results of the model compared to the results obtained by Eracleous, et. al.. Their analysis was performed using the Penn State ACIS simulator in conjuction with marx and xspec running in some sort of a loop.

Confidence contour figure

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This page was last updated May 31, 2002 by John E. Davis. To comment on it or the material presented here, send email to davis@space.mit.edu.
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