Any new data products (plots, tables, summaries of work done) should be posted here. I suggest a new titled section for each product or related products, and plots can be attached as PDFs (without a preview) or JPG/GIF/PNG images (which will show up inline). Plots should have some text summary about what they are. And we can link to these from the Outline and Action Items pages.
Contents:
17 Feb 2011 - ACIS peaks and sigmas
#anchor17 Feb 2011 - Catherine's attempt to duplicate Bev's monthly XIS plots for ACIS I3 (FI) and S3 (BI). Each data point is a fit to 0.1 years worth of cold ACIS calibration source data, filtered to include only the upper corner regions like XIS and grades G02346. After some testing as to which fitting procedures were less sensitive to background variations (important for ACIS probably not as much for XIS), and robust against low signal-to-noise data, I chose to use a Gaussian plus a linear background component (mpfitfun.pro with some limits to prevent non-physical fits).
24 Mar 2011 - CEG - The line normalizations that come out of the above fits. The first is a linear plot of the normalization versus time showing the obvious signature of radioactive decay.
Then comparing the data to the expected half-life of 2.737 years for Fe-55. There is clearly a slow reduction in quantum efficiency of 5-10% over the entire 11 years.
To confirm the source of the quantum efficiency reduction, here is the same plot, but for fits from the data in the lower corners of the CCD - closest to the framestore. There is no reduction in quantum efficiency for these data, indicating that the drop is due to increased CTI. Increased CTI leads to more charge trailing and more events morphing from acceptible to bad grades. CTI correction (the default in standard processing but not used here), would remove most of the QE drop.
May 4, 2011 - Bev
XIS peak location vs Cut Off Rigidity for recent times (201101).
while the last points of xis3 (FI) seem a bit odd, but neither the FI nor the BI are well fit with a line. The peak locations are not a function of the background variations.
Here are plots of the xis1 (BI) high energy count rate vs cut off rigidity to compare to the ACIS s3 (BI) high energy reject rates used to track the background levels for ACIS.
For January 2011:
For November 2006:
May 4, 2011 - Catherine
The plots from my IACHEC talk which are possibly paper-worthy plus some text.
The fractional change in the centroid of the Mn-Kalpha line as a function of time. The left panels are for the XIS detectors on Suzaku and the right panels are ACIS on Chandra. The top panels are representative front-illuminated devices (XIS3 and ACIS-I3) while the bottom are back-illuminated (XIS1 and ACIS-S3). The triangles indicate XIS data with charge injection turned on - ACIS does not have the ability to inject charge.
The change in the line centroid in the absence of any other gain or electronic changes, can be used as a proxy for the change in charge transfer inefficiency (CTI). Radiation damage increases CTI, which in turn decreases the line centroid for data far from the detector readout. The decrease in the line centroid for both XIS and ACIS is due to increasing CTI from radiation damage. Turning on the charge injection for XIS both improved the CTI and decreased the rate of change of CTI.
The rate of change for the two instruments is quite different. The front-illuminated device XIS3, even with charge injection turned on, has a centroid decrease that is roughly 5 times larger than ACIS-I3, while the back-illuminated device XIS1, with charge injection, has a centroid decrease that is 9 times larger than ACIS-S3. This difference may be due to the much warmer temperatures on XIS (-90C versus -120C), or to the different radiation environment encounter in low- and high-Earth orbit.
In addition, the XIS data appear much smoother than the ACIS data. This is due to the much higher and more variable particle background found in high-Earth orbit. These particles act as the primary source of sacrificial charge which can reduce the effective CTI. Due to the details of the charge trap time constants on ACIS (Grant et al. 2004? need to check this), the front-illuminated ACIS-I3 is more susceptible to sacrificial charge than ACIS-S3.
The FWHM of the Mn-Kalpha line as a function of time. Again, the two instruments behave differently. Due to the early damage from passage through the Earth's radiation belts, ACIS-I3 starts with a much higher level of CTI and larger line FWHM. The smaller initial XIS1 FWHM, as compared for ACIS-S3, is due to improvements in manufacturing back-illuminated devices. The XIS FWHM increase with time, while the ACIS FWHM are almost unchanging. Utilizing charge injection reduces the FWHM and the rate of FWHM increase.
The FWHM of a Mn-Kalpha line as a function of the line centroid. While CTI and FWHM should go hand-in-hand, the relationship is clearly complicated, possibly device- and environment-specific.
The counts in the Mn-Kalpha line as a function of time. Most obvious is the decline in counts due to the radioactive decay of the Fe-55 sources (half-life of 2.73 years). The ACIS source is much brighter, at least initially, but both sources suffer from increasingly low signal to noise as the missions continue.
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.)
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
and http://space.mit.edu.ezproxyberklee.flo.org/~cgrant/line 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.
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?)