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

Understand

The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to survey the gamma-ray sky 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 energy scale.
• However, the geomagnetic cutoff in 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 cutoff can be calculated with the aid of a numerical code tracing charged particles in the Earth's magnetic field.