CXO Project Science: ACIS Background - ACE SIS Correlation

These plots use the data from the Solar Isotope Spectrometer (SIS) on the ACE spacecraft. (This should not be confused with the other SIS that some of you might have heard of :-). The ACE data can be found by first going to ACE Browse Data and then selecting "Flexible Browse Data Interface" 252/1999 - current. On the following page select "ACE Browse Daily Averages", and then on the SIS row, select H,E.>10 and/or H,E.>30. (We also suggest a linear Y scaling).

We note that there is a flare in the SIS data on days 322, 323. We believe this was caused by ACE switching telemetry formats because of the Leonids and screwing up the data. (These are different data from a different team.)

Below we plot the ACE/SIS low energy (E>10 MeV) rate divided by the average value of 1.075. For ACIS, the I array average overflow rate normalized was by dividing by 146.3. Thus in both plots the absolute fractional change can be directly seen.

As can be seen, there is a high degree of correlation between the two rates over both the long and short terms. It is worth noting that the ACE/SIS data a daily average, whereas the ACIS is averaged for a given obsid typically lasting a few hours. Thus small differences are expected. The differences that occur between days 265 to 280 appears larger than one would expect from looking at the rest of the data.

We conclude that the ACIS overflow channel is highly correlated to the SIS low energy rate data. You should read the ACE/SIS documentation to find out more about the origan of these particles. Since the SIS rate is typically 1 ct/cm2/sec/ster/MeV and if we assume a 10 Mev bandpass, 2 pi steradians, 6.25 cm2, an ACE/SIS rate of 1 implies 390 events/sec should be impacting an ACIS chip. The average overflow rate is near 150 ct/sec. At this time it is unclear what the low energy cutoff really is, and what fraction of cosmic rays events are detected as actually detected as overflow events as opposed to being tossed due to bad grades.

Return to our ACIS page, go to the Chandra Project Science Page or to X-ray Astronomy at MSFC

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