The Large Area Telescope (LAT) team has reprocessed its entire Pass 7 dataset with up to date instrument calibrations. The reprocessed data, referred to as P7REP data, is now available from the FSSC's LAT Data Server as well as the FTP site in the form of weekly photon files. The reprocessed data provides a number of improvements over the original Pass 7 data, and is considered the best dataset for all types of LAT analysis. As of the release date (November 6, 2013) original Pass 7 data is no longer being served. However, existing original Pass 7 data has been archived and is available from the FSSC's FTP server.
Here we discuss the changes to the data and how they affect your analysis.
The reprocessed LAT data has applied new calibration constants to the data. These new calibrations better describe:
The major effects of applying these updated calibrations are: increases of up to a few percent in the energies of the events (depending on time and moderately on energy), and more accurate direction reconstructions at energies greater than ∼3 GeV. More details can be found here.
- the reduction of light yield from the LAT Calorimeter (CAL) logs versus time. This reflects the expected ∼1% per year signal loss due to crystals aging from irradiation onboard the satellite.
- the asymmetry in the light collection at the two ends of each calorimeter log.
- the time evolution of LAT Tracker (TKR) routine calibrations, i.e. hot and dead strips, signal energy scale.
A change in the reconstruction code provides a small improvement to calorimeter positions used in event-level analysis.
The LAT team has provided an updated version of the Galactic Diffuse model. As always, this model is designed to be used for point source analysis, but is not appropriate for the investigation of medium or large scale diffuse structures within the LAT data. A complete description of the new Galactic diffuse model is available here. This model is included in the current release (v9r32p5) of the Fermi Science Tools.
As is done for every data release, the LAT team has optimized the Instrument Response Functions (IRFs) and has revised the isotropic diffuse model to accurately reflect the improved performance of the P7REP data. The recommended IRFs for P7REP data analysis is P7_V15, and these have been released with the new version (v9r32p5) of the Fermi science tools. The updated isotropic models are also included in the new tools. In addition, you can download the new files from the LAT Background Models web page.
The precalculated diffuse response columns have been populated with values for the P7REP Source and P7REP Clean event classes. These precalculated diffuse responses were generated using the new Galactic diffuse model and class-specific isotropic models. If you choose to use any other diffuse models, you must use gtdiffrsp to calculate the diffuse response of those model prior to performing an unbinned likelihood analysis. This includes when you are including an extended source in your source model.
Weekly files for the P7REP data contain the Source, Clean, and dataclean event classes. These weekly data and spacecraft files are available at the ftp site, and can be retrieved with wget:
wget -m -P . -nH --cut-dirs=4 -np -e robots=off http://heasarc.gsfc.nasa.gov/FTP/fermi/data/lat/weekly/photon/
No data acquired after the November 6, 2013 release date will be processed using the original Pass 7 analysis.