Chandra Aspect Operations |
Old Analysis | New Analysis |
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Warm pixels with raw (median filtered) data and Bayesian blocks intervals
Warm Pixels Plotted Simultaneously Per Perigee Pass
Concatenated Data from Warm Pixels seen in 2002 and 2008/9.
Histograms of 'jump' fraction. Here "A" is the pixel count rate in e-/sec. The shape of the histogram of the 'jumps' as a fraction of the preceding count rate is slightly more broad for 2008/2009 data:
The dip in the middle of the above plot is due to the fact that the Bayes analysis is not sensitive to small jumps.
The following plot is a histogram of the durations of the intervals determined by the Bayesian Blocks analysis.
The differing slopes of the older and newer data in this duration histogram show that the 2008/2009 data appears to have more intervals of longer durations than the 2002 data. However, as the 2008/2009 data was collected over longer intervals (more full perigee pass data instead of the first obsid of a pass as used in 2002) it is not unexpected that there is a larger fraction of longer intervals. This does not rule out other possible contributing factors, such as operating temperature and accumulated radiation exposure.
As the CCD is exposed to incoming photons from the observed field and the fixed monitor window passes over areas of the sky, the light from dithering faint stars may contaminate these data. Ionizing radiation may also cause rate spikes. In both cases, the 'flickering' behavior is not inherent to the 'aberrant' pixel. Intervals which appear to be contaminated by stars have been manually marked and excluded from this analysis. See Pass 2008:357 for an example.
The Bayesian Blocks analysis cannot distinguish small magnitude flickering from noise, and thus this analysis is not sensitive to any such behavior.
The behavior of flickering pixels in the ACA CCD appears largely unchanged from 2002 to 2008/2009. No new behaviors have emerged and the CCD appears to be very stable. As mentioned, the appearance of more intervals of longer duration in the new data set is likely a result of the data collection strategy, but as other causes cannot be ruled out with this analysis, it may be valuable to repeat the data collection strategy used for this study in a future study.
As the collected data from 2008/2009 will be used to update the flickering pixel simulation library, it may also be valuable to gather data to update that library again in the future.
Last modified:01/13/11
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