The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

3.3. NON-GRB PRODUCTS 15
Figure 3.2: Representation of a detector plane histogram (DPH).
Descriptions of the formats of these products can be found in Appendix A (“BAT Data Formats”).
3.3.1 Survey Data
When not responding to a GRB trigger, the BAT is typically in survey mode. In this mode, the
event data from the array is collected into detector plane histograms (DPHs) by the flight software.
These histograms have an 80-bin spectrum for each detector in the array (i.e. 32k detectors ×
80 bins) during the integration period. The energy bin widths are variable from one energy bin to
the next, but have remained constant in time throughout the mission to date. Each survey DPH file
records the energy binning used (in the FITS file, there is an EBOUNDS extension which contains
this information). Figure 3.2 shows a representation of a survey DPH. It is three dimensional in
the sense that there are two spatial dimension and one spectral dimension.
Survey data have a crude energy correction applied on-board, but should usually be further
corrected using the task baterebin.
Typically, DPHs are integrated for five minutes, but this duration is not always the same.
In some cases longer or shorter durations have been used for operational reasons (e.g., telemetry
reduction or to get diagnostic information about the instrument). Also, survey intervals are always
terminated when the spacecraft begins a slew to a new target or when entering the SAA. Generally
speaking, the survey data cannot be used to search for time variations on time-scales shorter than
5 minutes.
Survey data is particularly difficult to analyze. It contains the longest sky integrations, and
thus is potentially the most sensitive to hard X-ray sources, but also contains the greatest number
of systematic error contributions. Currently the BAT team does not support general survey data
analysis.
3.3.2 Rate Data
The BAT produces continuous streams of rate data. While these may be used for scientific analysis,
they are also used by the on-board flight software for trigger searching and reporting the instrument
state of health.