Fermi Gamma-ray Space Telescope


Terms used in a definition that have their own glossary entry are italicized.

Term or Acronym Definition
ACD Anti-Coincidence Detector
Anti-Coincidence Detector (ACD) Scintillating tiles on the surface of the LAT that veto events resulting from charged particles form outside the LAT.
BATSE Burst And Transient Source Experiment—the gamma-ray burst detector on CGRO.
CAL Calorimeter
Calorimeter (CAL) Set of CsI logs at the bottom of the LAT that track, stop and record the energy of the electron-positron pair resulting from gamma rays that pair produce in the Tracker.
CGRO Compton Gamma Ray Observatory—gamma-ray mission that operated between 1991 and 2000.
Effective Area The number of photons detected divided by the source flux. The LAT effective area is a function of photon energy and inclination angle.
Event File FITS file with LAT events. Also called an FT1 file.
Field-of-View (FOV) Size of the region of the sky from which a detector accumulates data. The FOV has two definitions. One is the integral of the effective area over the sky divided by the on-axis effective area. The second is the total area of the sky over which useful data can be accumulated, even if the effective area is much less than the maximum.
FITS File File with a standard, self-defining structure. FITS files consist of one or more HDUs, an ASCII header and a data table. The header contains keywords identifying the file and HDU, or defining the data. The first HDU is used for images, and often does not include any data. FITS stands for 'Flexible Image Transport System.'
FOT Flight Operations Team—the staff at the MOC during the mission.
FTOOLS A package of tools to manipulate FITS files.
FOV Field-of-View
GCN Gamma-Ray Coordinates Network—system that receives and disseminates burst information.
HDU Basic unit of a FITS file consisting of an ASCII header and a data table. HDU stands for 'header and data unit.'
Inclination Angle The angle between the direction to a source and the LAT detector normal.
Instrument Response Function (IRF) Mapping between the quantities describing an incident photon (e.g., energy, direction) and the observables reported by an instrument.
IRF Instrument Response Function
LAT Large Area Telescope
Likelihood Probably of obtaining the data given a model.
Livetime Cube Array of the livetimes at a given LAT inclination angle at grid points over the entire sky. The livetime cube is stored in a FITS file.
MET Mission Elapsed Time
Mission Elapsed Time (MET) The number of seconds since midnight January 1, 2001 (UTC). MET is the time system used by all science software and data products.
MOC Mission Operations Center—the organization that controls the spacecraft. The MOC is housed at GSFC.
Photo-Multiplier Tube (PMT) A vacuum tube that emits a large electron current proportional to the small number of photons that enter the tube.
PMT Photo-Multiplier Tube
Point Spread Function (PSF) The distribution of the angular distance between a photon's actual and apparent positions. The LAT PSF is a function of photon energy and inclination angle.
PSF Point Spread Function
Region of Interest (ROI) Spatial region from which LAT photons are accumulated.
ROI Region of Interest
SAA South Atlantic Anomaly
SAE Standard Analysis Environment—the Fermi-specific data analysis tools. Also known as the Fermitools.
Source Region Spatial region whose gamma-ray sources are included in an analysis of LAT data. The Source Region should be larger than the Region of Interest.
South Atlantic Anomaly (SAA) Region over the south Atlantic where the trapped radiation is very large, requiring detectors to be turned off.
Spacecraft File FITS file with the time history of the spacecraft position, LAT orientation and LAT livetime. Also called an FT2 file.
Target of Opportunity (TOO) A target that is observed on a short timescale resulting from a change in the target (e.g., a flare).
TDRSS Tracking and Data Relay Satellite System
Test Statistic (TS) Statistic used to compare a model with a source to the model without the source (the null hypothesis). The TS is equal to -2 times the logarithm of the ratio of the likelihoods for the model with and without the source.
TKR Tracker
TOO Target of Opportunity
Tracking and Data Relay Satellite System (TDRSS) Constellation of geosynchronous satellites through which NASA spacecraft can send and receive data. An individual satellite is called a Tracking and Data Relay Satellite (TDRS).
Tracker (TKR) Component of the LAT consisting of tungsten foils in which gamma rays pair produce and Silicon Strip Detectors that track the passage of the resulting electron-positron pairs.
TS Test Statistic
Zenith Point on the sky opposite the earth.