In-flight measurement of the absolute energy scale of the Fermi Large Area Telescope

The LargeAreaTelescope (LAT) on-board the Fermi Gamma-ray Space Telescopeis a pair-conversion telescope designed to survey the gamma-raysky from 20 MeV to several hundreds of GeV. In this energy band there are no astronomical sources with sufficiently well known and sharp spectral features to allow an absolute calibration of the LAT energyscale. However, the geomagnetic cutoffin the cosmic ray electron-plus-positron (CRE) spectrum in low Earth orbit does provide such a spectral feature. The energy and spectral shape of this cutoffcan be calculated with the aid of a numerical code tracing charged particles in the Earth's magnetic field. By comparing the cutoffvalue with that measured by the LAT in different geomagnetic positions, we have obtained several calibration points between ∼6 and ∼13 GeV with an estimated uncertainty of ∼2%. An energy calibration with such high accuracy reduces the systematic uncertainty in LAT measurements of, for example, the spectral cutoffin the emission from gamma raypulsars.