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 - Contain a list of events organized in columns for each reconstructed event parameter. Two types of events files are provided with different event class pre-selections:
    • Photon File - Contains SOURCE-class events and columns with all information necessary (time, energy, position, etc.) for science analysis with Source, Clean, Ultraclean, or Ultracleanveto event classes.
    • Extended File - Contains TRANSIENT020E- and TRANSIENT015S-class events. The Extended File selection is a superset of the Photon File selection. These files also contain additional event-level quantities produced by the Level 1 analysis that are not available in the photon files.s
  • 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 - A bitfield indicating which event class selections a given event has passed. In Pass 8 the internal FITS format of this column has been changed from a 32-bit integer (TFORMn=J) to a 32-bit bit column (TFORMn=32X) and supports bitwise selections with the fselect FTOOL. Pass 8 populates a much larger number of bit values than Pass 7. Bits for the recommended event classes are bit 4 (P8R2_TRANSIENT020), bit 7 (P8R2_SOURCE), and bit 10 (P8R2_ULTRACLEANVETO).
  • EVENT_TYPE - A bitfield indicating which event type selections a given event has passed. This column is a 32-bit bit column (TFORMn=32X) and supports bitwise selections with fselect.
  • 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 an additional component (currently unused)
  • DIFRSP1 - Diffuse response for an additional component (currently unused)
  • DIFRSP2 - Diffuse response for an additional component (currently unused)
  • DIFRSP3 - Diffuse response for an additional component (currently unused)
  • 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:

  • WP8CTAllProb - The probability that the particle is a cosmic ray versus a gamma ray (0=CR-like, 1=gamma-like). This variable is used to define the background rejection filter for classes in the Standard and Extended event class hierarchies.
  • WP8CTAllBkProb - The probability that the particle is a front-entering versus a back-entering gamma ray (0=back-entering, 1=front-entering). This variable is used in the definition of classes in the Standard and Extended event class hierarchies.
  • WP8CTCalTkrProb - Same as WP8CTAllProb but evaluated using only CAL and TKR variables (no ACD). This variable is robust to pileup effects in the ACD. This variable is used to define the background rejection filter of classes in the "No-ACD" event class hierarchy.
  • WP8CTCalTkrBkProb - Same as WP8CTAllBkProb but evaluated using only CAL and TKR variables (no ACD). This variable is robust to pileup effects in the ACD. This variable is used in the definition of classes in the "No-ACD" event class hierarchy.
  • WP8CTBestEnergyProb - Probability that the error on the reconstructed energy of this event is smaller than average (0=poor reconstruction quality,1=good reconstruction quality). This variable is used to define the EDISP event type selections.
  • WP8CTPSFCore - Probability that the error on the reconstructed direction of this event is smaller than average (0=poor reconstruction quality,1=good reconstruction quality). This variable emphasizes the quality of events in the core of the PSF (68% containment angle). This variable is used to define the PSF event type selections.
  • WP8CTPSFTail - Probability that the error on the reconstructed direction of this event is smaller than average (0=poor reconstruction quality,1=good reconstruction quality). This variable emphasizes the quality of events in the tail of the PSF (95% containment angle).
  • WP8BestXDir - Analysis choice for the best X-direction cosine in the direction estimate.
  • WP8BestYDir - Analysis choice for the best Y-direction cosine in the direction estimate.
  • WP8BestZDir - Analysis choice for the best Z-direction cosine in the direction estimate.
  • WP8BestSXX - Analysis choice for the best XX slope element in the covariance matrix.
  • WP8BestSXY - Analysis choice for the best XY slope element in the covariance matrix.
  • WP8BestSYY - Analysis choice for the best YY slope element in the covariance matrix.
  • CalEnergyRaw - Measured energy in calorimeter after removal of pileup activity by the calorimeter clustering analysis. This variable is always equal to or less than the reconstructed energy.
  • 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).
  • PtPosx - X-component of the position vector of the spacecraft in Earth-centered inertial coordinates (same as SC_POSITION).
  • PtPosy - Y-component of the position vector of the spacecraft in Earth-centered inertial coordinates (same as SC_POSITION).
  • PtPosz - Z-component of the position vector of the spacecraft in Earth-centered inertial coordinates (same as SC_POSITION).

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 = GPS 13 microsecond timing anomaly. Can be used in standard analysis. May affect pulsar timing 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.
    • -3 = GRB 221009A. The standard IRFs do not properly describe the data. See the caveats about the data for this source.
  • 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
  • RA_SUN (degrees) - RA of the Sun at START
  • DEC_SUN (degrees) - Dec of the Sun at START
  • SC_VELOCITY (meters/second) - Three element array giving the spacecraft velocity in the same coordinate frame as SC_POSITION at START