This note documents the existence of a daily variation in the 30-70 keV background subtracted count rate. This is similar to the daily peak in the SAA dosage history. To determine the exact period, a power spectrum was calculated using the Lomb algorithm (Press et al.). The result was P=84571.4s, and the 30-70 keV light curves were folded on this period, using a common epoch of MJD = 50096.5970138888879, which is the time of a daily peak in the SAA dosage. The peak-to-peak amplitude of the variation ranges from ~0.3 to ~1.0 cts/s/3pcu. Contrary to earlier reports, there is no firm evidence for a variation in the 0-10 keV light curves.
Background subtracted light curves were accumulated in the 30-70 keV band for several observations that have become public (table 1), using the standard selection criteria for faint sources (table 2).
|Object||Obsid||30-70 keV light curve||30-70 keV light curve folded on period|
|(BKGD_THETAa < -70.0 or BKGD_THETA > 150.0) or TIME_SINCE_SAAb > 30.0 or TIME_SINCE_SAA < 0.0||Outside the South Atlantic Anomaly|
|PCUn_ON 0.5||PCUn fully on|
|ELV > 10.0||Angle between the satellite pointing direction and the Earth's horizon|
|OFFSET < 0.01||Angle between the satellite pointing direction and the nominal field of view|
|VpX1L, VpX1R < 6.25||Veto rate between the propane layer and the left/right halves of the Xenon layer|
|(VpX1L+VpX1R)/Q6c < 0.1||Normalized propane layer veto rate|