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The counts in the Mn-Kalpha line, after removing the 2.74 year Fe-55 half-life, as a function of time. (Not sure the paper needs this one.)

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June 21, 2011 - Catherine

Is the centroid of the Mn-Kalpha line in the upper corners of the CCD a reasonable proxy for CTI?

Yes, with caveats.

A change in CTI must change the accumulated charge loss and thus the pulseheight far from the framestore. A change in pulseheight, however, doesn't necessarily have to be related to CTI - it could also be due to a changes in the gain completely unrelated to radiation damage.

ACIS has a known slow change in the gain as a function of time as measured very close to the framestore where CTI should be negligible. For all of the CCDs except I0 and I2 it is monotonically decreasing at a rate of ~1 ADU/yr at 5.9 keV. (The gain change on I0 and I2 is pathological with jumps and annual trends that aren't relevant, so I don't use them here.) See http://space.mit.edu.ezproxyberklee.flo.org/~cgrant/gain^{Image Added} and http://space.mit.edu.ezproxyberklee.flo.org/~cgrant/line^{Image Added} for example plots.

The figure shows CTI versus the fractional change in pulseheight for the same time bins as all the above plots (CTI x 10^5; fractional change in pulseheight = pulseheight/pulseheight(t=0)). The top two panels are I3, the bottom panels are S3. On the left is the measured data. The CTI and the pulseheight do seem to be correlated with some additional messiness, particularly for I3. The data on the right has had a time-dependent correction applied to the pulseheight data to correct for the slow gain decrease mentioned above. The correction coefficient itself was fit by eye, finding the value that best reduced the I3 scatter. The correction is always less than 0.5% of the total pulseheight.

Image Added

I'm not sure how well the same type of correction could be made for XIS. What we may need to do is to beef up the error bars on the pulseheight data, to reflect our uncertainty on how well pulseheight works as a proxy for CTI.

(I am also ignoring the issue of changing serial CTI. For ACIS this is negligible, but might not be for Suzaku?)

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