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

• 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. By comparing the cutoff value 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 cutoff in the emission from gamma ray pulsars.

Subject headings

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

Keyword

Absolute energy scale

Cosmic-rays

Fermi Large Area Telescope

Geomagnetic cutoff

Absolute calibration

Astronomical sources

Calibration points

Cut-off value

Earth's magnetic field

Energy band

Energy calibration

Energy scale

Large area telescopes

Low earth orbits

Numerical code

Pair-conversion

Spectral feature

Spectral shapes

Systematic uncertainties

Calibration

Geomagnetism

Magnetic fields

Optical telescopes

Telescopes

Uncertainty analysis

Gamma rays

Publication and Content Type

ref (subject category)

art (subject category)