Fermi Large Area Telescope

Fermi contains a pair conversion telescope (Atwood et al 2009; ApJ, 697, 1071) that is sensitive to gamma-rays from about 20 MeV to 300 GeV. It is made up of a 4 x 4 matrix of identical towers, each one comprised of a tracker with layers of Silicon Strip Detectors (SSD) alternating with foils of high-Z converter (tungsten), and a calorimeter with logs of CsI arranged in a `hodoscopic' configuration so that the energy deposition is imaged in three dimensions. The array of towers is surrounded by an Anti-Coincidence Detector (ACD) made up of 89 plastic scintillator tiles with a 5 x 5 array on the top and 16 tiles on each of the 4 sides. A gamma-ray passes through the ACD with small probability of interaction and can convert into an electron-positron pair that is tracked in the SSD. The energy of a photon below about 100 MeV can be estimated by multiple-Coulomb scattering of the electrons in the tungsten converters and SSD. Energies of higher-energy photons are measured by `total' absorption or modeling the shower profile in the calorimeter. In the standard Fermi sky-survey mode the detector axis rocks about 50 degrees (35 degrees prior to September 3, 2009) from the zenith each orbit; therefore at most times during the year the Sun can be observed for only ~20-40 min every two orbits. With the LAT`s large aperture (2.4 sr) and effective area this provides the capability to sensitively monitor the Sun with a duty cycle of 15-20%. In response to a 'burst' trigger from the GBM during a high-energy solar flare Fermi can autonomously be pointed at the Sun for up to 5 hours. In addition when the Sun is in a high-state of activity a Target of Opportunity can be declared during which time the spacecraft remains in the rocking location most favorable to solar observations. During these times ~30-40 minute observations can be made every orbit.

Events not accompanied by an energy loss of 800 keV in the ACD (equivalent to 45% of the amplitude of a minimizing ionizing singly-charged particle traversing an ACD tile) are telemetered to the ground for further analysis. Thus the telemetered events mostly include what appear to be neutral particles or photons. Ackermann et al. (2012 ApJ 745, 144) presented a detailed description of the latest solar flare analysis procedures and instrument response functions for the telemetered LAT data. This level of processing, identified as Pass 7, optimized different event classes, increased detection efficiency below ~300 MeV, and took into account various on-orbit effects.