LAT Data Products

The LAT astrophysical data analysis that you will perform with the GLAST science tools begins with a list of events 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:

Also see: Other Files

Origin of the Photon Data

This Level 1 data (i.e., the FT1 and FT2 filetypes) result from processing the Level 0 data downlinked from the GLAST spacecraft. For the LAT, 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).
  • Characterizing the event's relevant physical parameters (e.g., direction, energy).

Notes:

  • Hits. Hits are signals resulting from the interaction of an event or its products with the various detectors in the LAT.
  • Merit Ntuple. The characterization of an event results in a set of ~200 parameter values called the Merit Ntuple.
  • Most 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 astrophysical photons have been extracted from the event data to form the Photon Summary File you will be using. However, it is always possible to go back to the full event dataset to examine the parameters and events that are not included in the Photon Summary files.

Contents of the Photon Summary Files

For each astrophysical photon the Photon Summary File contains the following information (the list does not include some of the more esoteric quantities):

  • Energy
   Apparent energy of the event, in MeV.
  • RA
   Right Ascension (J2000) of the photon's apparent origin, in degrees.
  • Dec
   Declination (J2000) of the photon's apparent origin, in degrees.
  • Theta
   Inclination angle, the angle from the LAT's normal to the photon's apparent origin, in degrees.
  • Phi
   Azimuthal angle, the angle of the photon's apparent origin around the LAT's normal, in degrees.
  • Zenith Angle
  

Angle of the photon's apparent origin to the Earth-spacecraft vector, in degrees
  • Earth Azimuth Angle
   Angle of the photon's apparent origin around the Earth-spacecraft vector, in degrees.
  • Time
   Mission elapsed time, in seconds.
  • Event ID
   ID number of the original event; this ID number allows you to look up all the photon's parameter values.
  • Reconstruction Version
   Version of the reconstruction software.
  • Calibration Version
   Version of the calibration tables.
  • Classification Probabilities
   Probabilities that various quantities are measured well.
  • Conversion Point
   Point in LAT where the photon pair-produced.
  • Conversion Layer
   Layer of the LAT's TKR where the photon pair-produced.
  • Pulse Phase
   Event's pulsar pulse phase; added during the analysis of a specific pulsar.
  • Orbital Phase
   Event's binary phase; added during the analysis of a specific binary system.
   

Contents of the Pointing and Livetime History Files

Pointing and Livetime History Files contain the following information for 30 second intervals.

  • Start Time
   Beginning of the interval in Mission Elapsed Time (MET).
  • Stop Time
   End of the interval in Mission Elapsed Time.
  • Positions
   Position and orientation of the spacecraft and the LAT at the beginning of the interval in various coordinates.
  • McIlwain Parameters
   Two parameters that describe the strength and gradient of the Earth's magnetic field at the spacecraft.
  • SAA Flag
   Indicates whether the LAT is off because GLAST is in the high radiation field of the South Atlantic Anomaly.
  • LAT Mode
   LAT's operational mode during the interval.
  • Livetime
   Detector's livetime.
  • Deadtime
   Detector's deadtime since the beginning of the mission.
   

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 product of the analysis.

  • Instrument Response Functions
   LAT's response will be characterized by a number of functions with empirically determined parameters. The science tools will access files with the parameters appropriate for a particular time period.
  • Diffuse Emission Map
   LAT will detect point sources on top of the very bright diffuse emission from the Galaxy and extragalactic sources. The LAT team will provide a model of this diffuse emission, but the user will be able to submit his/her own.
  • Pulsar Ephemerides
   GLAST tools will include a database of ephemerides of the pulsars that the LAT is most likely to detect.
  • Source Definition
  

Trial and fitted source models will be stored in XML files (one of the few non-FITS files).
  • Binned Spectra
   LAT photon data can be binned into spectra stored in the common PHA format; this will be a common gamma-ray burst analysis.
  • Bin Definitions
   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.
   

 

 

Owned by: David Band dband@milkyway.gsfc.nasa.gov

 

Last updated by: Chuck Patterson 02/23/2006