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Fermi Gamma-ray Space Telescope

Cicerone: Data - GBM Data Products

Overview

The GBM astrophysical data analysis that you will perform with the Fermitools will focus on spectral analysis of gamma-ray bursts, and as with the LAT analysis, will begin with a list of counts. Remember that the GBM consists of 12 NaI detectors (8-1000 keV) and 2 BGO detectors (150 keV to 30 MeV).

The burst files and the daily files both include CTIME and CSPEC, files with counts binned with different time and spectral resolution. The time and energy binning described below are the current defaults, but both can be redefined in tables read by the flight software, and thus may vary during the mission.

Burst Files

The GBM team provides a series of files for each burst. This first set is crucial for spectral analysis.

  • Time-Tagged Events (TTE)—lists of the counts in 128 energy channels from each GBM detector. These data start at 500,000 events prior to the burst (typically 15-30 seconds), and continue for 300 seconds after the trigger time, unless interrupted by an SAA entry.
  • Detector Response Matrices (DRMs)—the detector response mapping the input energy into the apparent count energy, provided for each detector with 128 energy channels. The GBM team will provide the files, but a tool will eventually be provided so that users can create their own files.

The following set of files are also provided for each burst. However, they are not crucial for standard spectral analysis using the Fermitools.

  • Catalog entry—the GBM team will calculate certain standard quantities, such as duration and fluence, and lightcurves of the spectral parameters. An initial catalog file will be provided with the other burst data products immediately after the burst, and updates will be provided later.
  • CTIME and CSPEC (burst versions)—series of spectra with different temporal and spectral resolution from 4000 seconds before, to 4000 seconds after, the burst will be included. As currently planned, CTIME provides 8 channel spectra every 0.256 seconds and CSPEC 128 channel spectra every 4.096 seconds, with a higher time resolution of 1.024 s immediately after the trigger.
  • TRIGDAT—the GBM team will package the burst alert telemetry, the information downlinked immediately after a burst.

Daily Files

The GBM team will provide a series of files every day.

  • CTIME and CSPEC (daily version)—series of spectra with different temporal and spectral resolution, one per detector, each covering one day. Again, as currently planned, CTIME provides 8 channel spectra every 0.256 seconds and CSPEC 128 channel spectra every 4.096 seconds.
  • TTE (continuous version)—event data for each detector with a time precise to 2 microseconds, and a number corresponding to 1 of 128 energy channels. This continuous TTE data collection mode was implemented for select parts of the Fermi orbit in July 2010. In November 2012 GBM entered a continuous TTE data collection mode over the entire orbit. The downlink schedule determines how many data files are produced each day.
  • Calibration and Housekeeping Files

Periodic Files

The GBM team will provide a series of files periodically.

  • Catalogs—the burst catalog entries will be updated.
  • Calibration—some calibration parameters will change slowly and will only have to be updated occasionally. Similarly, the modeling of the instrument response will be updated as the understanding of the detectors improves.

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