An Empirical Background Model for the NICER X-ray Timing Instrument

Understand

NICER has a comparatively low background rate, but it is highly variable, and its spectrum must be predicted using measurements unaffected by the science target.

• We describe an empirical, three-parameter model based on observations of seven pointing directions that are void of detectable sources.
• An examination of 3556 good time intervals (GTIs), averaging 570 s, yields a median rate (0.4-12 keV; 50 detectors) of 0.87 c/s, but in 5 percent (1 percent) of cases, the rate exceeds 10 (300) c/s.
• Model residuals persist at 20-30 percent of the initial rate for the brightest GTIs, implying one or more missing model parameters.