Fermi Gamma-ray Space Telescope

Cicerone: Data — LAT Data Products

Overview

The LAT astrophysical data analysis that you will perform with the Fermi science tools begins with a list of counts that have been identified as resulting from astrophysical photons. The analysis requires information about where the LAT was pointing and what was the observing efficiency. Therefore you will extract and manipulate two types of FITS files: the 'photon file' and the 'spacecraft file.' These two filetypes have been given the file extensons .FT1 and .FT2, respectively. An additional 'extended file' is also available that provides the same photon list with a few additional parameters that are not required for analysis with the science tools.

Origin of the Photon Data

These two filetypes result from the processing of the data downlinked from the Fermi spacecraft (considered to be 'Level 0' data), and are therefore regarded as 'Level 1' data. For the LAT the Level 1 processing involves reconstructing the interaction of the event in the LAT from the 'hits' in the various parts of the LAT, identifying the type of event (e.g., astrophysical photon), and characterizing the event's relevant physical parameters (e.g., direction, energy). 'Hits' are the signals that result from the interaction of the event or its products with the various components of the LAT. The characterization of an event results in a set of ~200 parameter values forming the 'merit n-tuple.' Most of the events are not astrophysical photons, and most of the parameters describing an event are not relevant for the data analysis carried out by the science tools. Therefore, a small set of parameters for the counts considered astrophysical photons have been extracted from the event data to form the event file you will use for most astrophysical analysis. This optimal screened event list based on our current best knowledge of the instrument comprises our basic archived LAT data product.

Nonetheless, the full event dataset will always be available for users who wish to examine the full set of parameters of the astrophysical counts and also many events that are not included in the event files. The scientific community will be able to extract both file types from the databases of the FSSC.

Photon Classification

In reconstructing the events from the hits in the LAT, the LAT instrument team will make various cuts that will classify the events based on the probability that they result from photons and the quality of the reconstruction. The events will be separated into various event classes; each class will be characterized by its own set of instrument response functions. The reconstruction methodology and the event class cuts have evolved and are likely to continue so.

The current event classes are a nested heirarchy in which the higher probability photon selections are subsets of the less restrictive selections. Higher probability photon selections have smaller effective areas, narrower point spread functions (PSF), and lower fractions of background counts. The most restrictive selection (the 'DataClean' class in the Pass 6 reconstruction) is ideal for analysis of large regions that are more sensitive to spectral features caused by instrumental backgrounds. Lower probability photon selections provide larger effective areas, particularly at low energy, at the expense of allowing more background contamination. An intermediate selection (the 'Diffuse' class in Pass 6) that provides enhanced effective area but still limits the extent of the PSF and the level of background contamination is more favorable for analysis of moderately extended sources and point sources. The loosest selection criteria (the 'Transient' class in Pass 6) is designed for short duration events, such as gamma-ray bursts, and timing studies that benefit from increased photon statistics while tolerating a higher background fraction and broader PSF. In this nested scheme, the 'DataClean' class is a subset of the 'Diffuse' class, and the 'Diffuse' class is a subset of the 'Transient' class.

Each analysis class has its own effective area and point spread function, and the PSF varies with energy. You will need to indicate which Instrument Response Function set corresponds to your analysis selection when using the science tools. The tools will apply the instrument response functions appropriately to a given count in analyzing that count.

Data File Contents

For each astrophysical photon the LAT photon file contains the information necessary for science analysis. This includes the energy of the event, the position, as well as information about the quality of the event reconstruction. The extended data file contains the same photon data, plus information helpful for understanding the results of the Level 1 analysis. However, this information is unlikely to be useful for the typical analysis. Finally, there is the spacecraft file that contains spacecraft position and orientation information for 30 second intervals. (Some intervals at the ends of a file may be shorter than 30 seconds.) A description of each column in these files has been provided.

Note that during the analysis process, additional information may be added to a FITS file, either as a keyword or an additional column, as long as its name differs from that of an existing type of data. Thus you might find additional information in some event files.

Other Files

In analyzing the LAT data you will use other files, some of which the tools access without your intervention, some of which are intermediate products of the analysis.

  • Instrument Response Functions (IRFs)—the LAT's response is be characterized by a number of functions with empirically determined parameters. The science tools access files with the parameters appropriate for a particular time period.
  • Diffuse Emission Map—the LAT detects point sources on top of the bright diffuse emission from the Galaxy and extragalactic sources. The LAT team will provide one or more models of this diffuse emission, and the user will be able to substitute his/her own.
  • Pulsar Ephemerides—a database of ephemerides of the pulsars that the LAT might detect.
  • Livetime Cubes—over a specified time range, a livetime cube provides the livetime the LAT observed a region of the sky at a given inclination angle (see the discussion in 'Precomputation of Likelihood Quantities'). Because the computation of these data is very time consuming, they are provided for specific time ranges.
  • Source Definition—trial and fitted source models are stored in XML files (one of the few non-FITS files).
  • Binned Spectra—the LAT photon data can be binned into spectra stored in the common 'PHA' format; this will be common for gamma-ray burst analysis.
  • Bin Definitions—the grids used to bin spectra in time and energy can be input through FITS files.
  • Response Matrices—to analyze binned spectra, the IRFs must be integrated over space and stored in the common 'RSP' format.

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