Dependence of Background Rejection Efficiency on Event Grade
Selection
from the SI Calibration Report,
1/15/99, pp. 231-234
An important consideration in choosing ACIS event selection criteria is the efficiency of background rejection. To illustrate this point, we present in Tables 1, 2, and 3 the fraction of background events in various grades. For conciseness, we list only the standard ASCA grades, and ACIS grade 255. A similar analysis should be performed (presumably by the AXAF Science Center) for all 255 ACIS grades before a final decision is made on the definition of standard ACIS grades. Of course, the raw XRCF background event lists we have compiled are available to ASC for this purpose.
A cursory analysis of these tables shows, for example, that in device I3 91.21% of background events were ACIS bit map grade 255 events (i.e. all eight neighboring pixels were above the split threshold). However, only 4.14% of the background events in S3 were ACIS grade 255 events. Thus, a large majority of the background events in a frontside device could be rejected simply by filtering ACIS grade 255 events, but only a very small percentage of the background events in a backside device would be rejected. If one were to reject only ACIS grade 255 events, the total on-orbit background rate for six devices should be ~40 ct s-1, which is about 22% of the telemetry bandwidth.
Table 1: Fraction of Background Events per Event Grade Measured at XRCF in the Passband 0.1-10 keV for Detectors I0, I1, I2, and I3.
*All event grades except B255 are ASCA grades. Here B255 stands for ACIS bit grade code 255 (i.e., all pixels above threshold).
Table 2: Fraction of Background Events per Event Grade Measured at XRCF in the Passband 0.1-10 keV for Detectors S0, S2, S4, and S5.
*All event grades except B255 are ASCA grades. Here B255 stands for ACIS bit grade code 255 (i.e., all pixels above threshold).
Table 3: Fraction of Background Events per Event Grade Measured at XRCF in the Passband 0.1-10 keV for Detectors S1 and S3.
Postscript tables:
