Fermi Gamma-ray Space Telescope

Bad Time Intervals

Bad Time Intervals (BTIs) are periods when the data are not useable for scientific analysis. These are defined differently for the LAT and GBM.

LAT

With the beginning of the solar activity in early 2009, it became clear that solar flares can cause significant X-ray pileup in the anti-coincidence detector (ACD), clearly visible in many low-level ACD quantities. The on-board ACD veto electronics are fast enough that this does not cause any change in the efficiency of the trigger and/or that of the gamma filter (i.e., there is essentially no loss of gamma rays onboard). However, the effects of the pileup are potentially much more severe at the level of the event selection on the ground, where the slow signals are used, since the additional activity in the ACD can cause gamma-rays to be misclassified as background.

From the point of view of the data analysis, the most relevant implication is that there are time intervals in which the standard instrument response functions (IRFs) do not accurately represent the detector, and, therefore, the results of the likelihood analysis are potentially unreliable. These BTIs are systematically identified and flagged. All LAT gamma-ray data automatically have the DATA_QUAL field in the spacecraft pointing and live time history file (i.e., spacecraft file) set to 1 by the data processing system and are immediately exported to the FSSC where they become publicly available. The potential for BTIs is reviewed only after the fact. This is because a high percentage of the LAT data is good and making them publicly available as quickly as possible is a priority.

For each suspect time period, the entire information available is reviewed manually. If there is a significant loss of efficiency, the corresponding time period is marked as BAD, and new 30-second and 1-second spacecraft files are generated with the DATA_QUAL field set to -1 or lower in the corresponding time bins. These new files are then exported to the FSSC and supersede the original files.

GBM

The bad time intervals (BTIs) for GBM data are identified by team members on an as-needed basis. Causes for BTI flagging includes TTE data throttling, detector saturation, and pulse pile-up effects.

Before December 2012, GBM has adjusted Low Level Threshold (LLT) settings to prevent excessive data volume due to solar flares. A list of times for the modified setting is available in this table. Since 2012, GBM flight software implemented data throttling that minimizes triggering from the same transient event such as solar flare and soft gamma-ray repeater.

A list of detector data gap due to safe hold or thermal issues are also available in this table.

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