...
Trend of the XIS NXB from night Earth data, extracting a 512x512 box from the chip center, using only COR2>6 data with normal SCI on (XIS1 at 2 keV level). XIS1 has a clear downward slope of -0.003 cts/s/yr +/- 20%, or about 3% per year. XIS0,3 are consistent with no change. Perhaps this is actually a gain issue with XIS1.
...