Does the background vary?
Yes. Significantly. See the 'Background' category in the Data Analysis Frequently Asked Questions. All components of the background, except for the particle background, vary considerably. The cosmic background in both the hard and soft bands vary by at least a factor of five over the sky. In addition, they vary independently, resulting in a significant change in the spectral distribution of the cosmic background. The non-cosmic background components can have even greater variation. The intensity of scattered solar X-rays can vary from zero for night-side observations to more than 50 counts/s for day-side observations early in the mission. Auroral X-rays can produce count rates well over 200 counts/s but are in general zero. Long-term enhancements, which mostly affect the 1/4 keV band, can vary between zero and about five counts/s. In addition, the lowest few channels are affected by afterpulses, which are time dependent.
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