Eur. Phys. J. C (2013) 73: 2578
https://doi.org/10.1140/epjc/s10052-013-2578-4

Regular Article - Theoretical Physics

Fermi-LAT gamma-ray signal from Earth limb, systematic detector effects and their implications for the 130 GeV gamma-ray excess

¹ National Institute of Chemical Physics and Biophysics, Ravala 10, 10143, Tallinn, Estonia
² Helsinki Institute of Physics, P.O. Box 64, 00014, Helsinki, Finland
³ Institute of Physics, University of Tartu, Tartu, Estonia
⁴ Tartu Observatory, Observatooriumi 1, Tõravere, 61602, Estonia

^* e-mail: andi.hektor@cern.ch

Received: 6 June 2013
Revised: 28 August 2013
Published online: 1 October 2013

Abstract

We look for possible spectral features and systematic effects in the Fermi LAT publicly available high-energy gamma-ray data by studying photons from the Galactic center, nearby galaxy clusters, nearby brightest galaxies, AGNs, unassociated sources, hydrogen clouds and from the Earth limb. Apart from the already known 130 GeV gamma-ray excesses from the first two sources, we find no statistically significant excesses from any of the cosmological sources nor from any control region. Therefore our main effort goes to the study of gamma rays appearing from the Earth limb. In the energy range of 30 to 200 GeV the Earth limb gamma-ray spectrum follows a power-law with spectral index 2.86±0.05 at 95 % CL, in a good agreement with the PAMELA measurement of the cosmic ray proton spectral index of 2.82–2.85, confirming the physical origin of the limb gamma-rays. In subsets of the Earth limb data at small photon incidence angle spectral features occur, including a feature at 130 GeV. We observe a systematic ∼2σ-level difference in the Earth limb spectra with small and large incidence angles. The behavior of those spectral features as well as the background indicates that those may be statistical fluctuations or complicated unknown systematic effects of the Fermi LAT. In the latter case, only the Fermi LAT Collaboration can give the final answer having access to raw data and all details of the reconstruction.