4.5.1 Classification of Events and Grade
To eliminate events due to charged particles and to attain the expected energy resolution, X-ray events in the depletion layer from each readout are identified and classified. The first step in finding an X-ray event is to apply the criterion that the central pixel of a 3 x 3 sq-pixel block must have the highest pulse height. In FAINT Mode, if the pulse height of a pixel which satisfies this criterion is higher than the event threshold, the pulse heights of the 3 x 3 sq-pixel block will be sent to the ground.
In BRIGHT Mode, the eight pixels around the central pixel will be compared with the split event threshold which is lower than the event threshold. Based on the distribution of the pixels which have a pulse height higher than the split event threshold, the event is classified, i.e., it is assigned a grade. With this scheme, all the basic charge distributions found by experiment can be included.
The eight grades (0-7) are extensions of four basic groups: S (single), P (single-sided split; subdivided into Vertical, Left and Rights), L (L-shaped) and Q (square-shaped). The extensions arise because of the existence, in some events, of corner pixels which do not belong to the block since they touch the distributions only at the corners (so-called `detached' corners). Usually, charge from an X-ray interaction (less than, say, 5 keV) is split into 1 or 2 pixels, i.e., S events (grades 0-1) and P events (grades 2-5). However, grade-1 events, which consist of single events with one or more detached corners, should be ignored because most are particle events (it is hard for an X-ray event to split charge diagonally). A sizable fraction of higher energy X-rays (5-10 keV), which tend to be absorbed deeper in the CCDs, are detected as L or Q (grade-6) events, while grade 7 events are dominated by particle events. Since each pixel included in the final PHA calculation contributes its own read-out-noise, grade 0 events have the best resolution and grade 6 the worst. The SIS team has concentrated on a combination of grades 0, 2, 3 and 4; this combination is therefore the best-calibrated. In some instances, however, it may make sense to add grade 6 events (to increase signal-to-noise at high energies) or to concentrate on grade 0.
Between launch and 1993 November 30 07:30 UT, on-board BRIGHT mode telemetry included events with grades 0-4; since then, events with grades 0-6 are telemetered in BRIGHT mode. FAINT mode telemetry contain all events including grade 7; it is customary to throw out grade 7 events during the conversion to BRIGHT or BRIGHT2 data.
Note that in FAST mode, a simpler version of grade is used: FAST mode grade 0 corresponds to BRIGHT mode grades 0 and 2, i.e., to good data. FAST mode grade 1 corresponds to all other BRIGHT mode grades, i.e., to the remaining good plus the bad grades.
