November 9, 1999

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).

Table 1: Light curves

Object Obsid 30-70 keV light curve 30-70 keV light curve folded on period
NGC7469 P10315 10315_lc.jpg 10315_fold.jpg
Coma Cluster P10368 10368_lc.jpg 10368_fold.jpg
IRAS18325-5926 P30229 30229_lc.jpg 30229_fold.jpg
IRAS18325-5926 P30405 30405_lc.jpg 30405_fold.jpg
MCG-6-30-15 P20310 20310_lc.jpg 20310_fold.jpg
NGC3516 P20316 20316_lc.jpg 20316_fold.jpg
NGC4051 P20318 20318_lc.jpg 20318_fold.jpg
NGC4051 P30225 30225_lc.jpg 30225_fold.jpg
PDS456 P30247 30247_lc.jpg 30247_fold.jpg
4-day background P30801 30801_lc.jpg 30801_fold.jpg

Table 2: Selection Criteria

Criterion Description
(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

a Longitude of the ascending node of the satellite's orbit
b Time since SAA passage
c Veto rate between six of the eight anode chains