Fermi Gamma-ray Space Telescope

Cicerone: Data — LAT Data Files - Column Descriptions

LAT File Types

There are two LAT data filetypes resulting from the Level 1 processing of the data downlinked from the Fermi spacecraft. This document describes the information contained in each column of those files. The Cicerone contains a detailed explanation of where this data originates. The two filetypes are:

  • Events Files - The two types of events files contain the same number of events
    • Photon File - contains all information necessary (time, energy, position, etc.) for science analysis with Source, Clean, or Ultraclean event classes
    • Extended File - contains all photon data (including Transient class), plus additional quantities produced by the Level 1 analysis
  • Spacecraft File - contains spacecraft location/orientation information necessary for data analysis

Photon File

For each event the photon file contains the following information:

  • ENERGY (MeV) - Reconstructed energy of the event
  • RA (degrees) - Reconstructed direction of the event in Right Ascension
  • DEC (degrees) - Reconstructed direction of the event in Declination
  • L (degrees) - Reconstructed direction of the event in Galactic Longitude
  • B (degrees) - Reconstructed direction of the event in Galactic Latitude
  • THETA (degrees) - Reconstructed angle of incidence of the event with respect to the LAT boresight (+Z axis of the spacecraft - the line normal to the top surface of the LAT)
  • PHI (degrees) - Reconstructed angle of incidence of the event with respect to the +X axis (the line normal to the sun-facing side of the spacecraft)
  • ZENITH_ANGLE (degrees) - Angle between the reconstructed event direction and the zenith line (originates at the center of the Earth and passes through the center of mass of the spacecraft)
  • EARTH_AZIMUTH_ANGLE (degrees) - Angle of the reconstructed event direction with respect to North (line from spacecraft origin to north celestial pole) as projected onto a plane normal to the zenith. The angle is measured in degrees east of north, such that 90 degrees indicates that the event originated from the west.
  • TIME (seconds) - Mission elapsed time when the event was detected (MET is the total number of seconds since 00:00:00 on January 1, 2001 UTC)
  • EVENT_ID - Sequence number for the event in the LAT data acquisition period
  • RUN_ID - Unique identifier for each LAT data acquisition period
  • RECON_VERSION - Version of event reconstruction software in use at the time the event was detected
  • CALIB_VERSION (3-element array) - Version of the calibration tables for the ACD, CAL, and TKR (in that order) in use at the time the event was detected. (This column is currently unused)
  • EVENT_CLASS - For Pass 7 analysis this is a bitfield indicating which selections a given event has passed. Many of the bits are for unused or as yet unvalidated selections. The only relevent bits are for P7TRANSIENT(bit 0), P7SOURCE(bit 2), P7CLEAN(bit 3) and P7ULTRACLEAN(bit 4). Note that for Pass 6 data this is an integer, with values 1-4 for Transient,Source,Diffuse and DataClean event classes respectively.
  • CONVERSION_TYPE - Indicates whether the event induced pair production in the front (thin) layers or the back (thick) layers of the tracker (front=0, back=1)
  • LIVETIME (seconds) - A short-term measure of accumulated livetime of the LAT. This value can have gaps and it resets every few seconds. For large time intervals, the LIVETIME documented in the spacecraft file is correct. However, for short time intervals, this LIVETIME value can be compared between two events to gauge the fraction of dead time.
  • DIFRSP0 - Diffuse response for extragalactic component for P7SOURCE class events and response functions
  • DIFRSP1 - Diffuse response for galactic component for P7SOURCE class events and response functions
  • DIFRSP2 - Diffuse response for extragalactic component for P7CLEAN class events and response functions
  • DIFRSP3 - Diffuse response for galactic component for P7CLEAN class events and response functions
  • DIFRSP4 - Diffuse response for an additional component (currently unused)

Extended File

For each event the extended file contains the same columns as the photon file above. In addition it adds the following information:

  • CTBBestEnergyProb - The probability that the best energy chosen from the three energy estimators is correct (based on a Classification Tree analysis)
  • CTBBestEnergyRatio - The ratio of the most probable reconstructed energy with the raw energy estimate from the calorimeter subsystem, If the event missed the calorimeter, this is the reconstructed energy of the event.
  • CTBBestSXX - Analysis choice for the best XX slope element in the covariance matrix
  • CTBBestSXY - Analysis choice for the best XY slope element in the covariance matrix
  • CTBBestSYY - Analysis choice for the best YY slope element in the covariance matrix
  • CTBBestXDir - Analysis choice for the best X-direction cosine in the direction estimate
  • CTBBestYDir - Analysis choice for the best Y-direction cosine in the direction estimate
  • CTBBestZDir - Analysis choice for the best Z-direction cosine in the direction estimate
  • CTBCORE - The probability that the direction estimate is good (based on a Classification Tree analysis)
  • CTBClassLevel - This value is currently a copy of the information provided in EVENT_CLASS.
  • PtAlt - Spacecraft altitude at the time of the event
  • PtDecx - Declination of the spacecraft +X-axis at the time of the event
  • PtLat - Ground point latitude at the time of the event
  • PtLon - Ground point longitude at the time of the event
  • PtMagLat - Magnetic latitude at the time of the event
  • PtRax - Right Ascension of the spacecraft +X-axis at the time of the event
  • PtRaz - Right Ascension of the spacecraft +Z-axis at the time of the event
  • PtSCzenith - Angle of the spacecraft Z-axis from zenith (positive values indicate a rock toward the north)
  • Tkr1FirstLayer - First tracker layer to show evidence of a particle hit for the best track reconstruction (tracker layers are 0-17 where 0 is closest to the CAL).

Spacecraft File

The spacecraft file contains the following information for 30 second intervals (some intervals may be shorter):

  • START (seconds) - Mission Elapsed Time of start of interval
  • STOP (seconds) - Mission Elapsed Time of end of interval
  • SC_POSITION (meters) - Three element array giving the position (x, y, z) of spacecraft in inertial (ECI) coordinates at START
  • LAT_GEO (degrees) - Ground point latitude at START
  • LON_GEO (degrees) - Ground point longitude at START
  • RAD_GEO (meters) - Spacecraft altitude at START
  • RA_ZENITH (degrees) - RA of zenith at START
  • DEC_ZENITH (degrees) - DEC of zenith at START
  • B_MCILWAIN (Gauss) - McIlwain B parameter, magnitude of the magnetic field at START
  • L_MCILWAIN (Earth radii) - McIlwain L parameter, distance/shell value at START
  • GEOMAG_LAT (degrees) - Geomagnetic latitude at START
  • IN_SAA (T/F) - Whether the spacecraft was in the SAA at START
  • RA_SCZ (degrees) - viewing direction at START (RA of S/C +z axis)
  • DEC_SCZ (degrees) - viewing direction at START (Dec of S/C +z axis)
  • RA_SCX (degrees) - viewing direction at START (RA of S/C +x axis)
  • DEC_SCX (degrees) - viewing direction at START (Dec of S/C +x axis)
  • RA_NPOLE (degrees) - RA of north orbital pole at START
  • DEC_NPOLE (degrees) - Dec of north orbital pole at START
  • ROCK_ANGLE (degrees) - Angle of the spacecraft Z-axis from zenith (positive values indicate a rock toward the north)
  • LAT_MODE - Spacecraft GNC mode, where the 3 nominal modes are 3(inertial point), 4 (Maneuver) and 5 (zenithpoint/survey). Other modes include 1 and 2 (capture and sunpoint - rarely used) and 6 and 7 (reentry modes).
  • LAT_CONFIG - Flag for the configuration of the LAT (1 = nominal science configuration, 0 = not recommended for analysis)
  • DATA_QUAL - Signed integer value indicating the quality of the LAT data, with the following meanings:
    • 0 = Bad data. Do not use.
    • 1 = Good data. Use for normal science.
    • 2 = Bright GRB event. Can use in standard analysis. Excluded from catalog analysis.
    • -1 = Solar Flare. The standard IRFs do not properly describe the data.
    • -2 = Particle event. The standard IRFs do not properly describe the data.
  • LIVETIME (seconds) - Accumulated livetime of the LAT during the interval from START to STOP
  • QSJ_1 - First component of SC attitude quaternion
  • QSJ_2 - Second component of SC attitude quaternion
  • QSJ_3 - Third component of SC attitude quaternion
  • QSJ_4 - Fourth component of SC attitude quaternion