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

TL;DR

This study analyzes Fermi-LAT gamma-ray data from various sources, focusing on Earth Limb photons, to investigate spectral features and systematic effects, especially around 130 GeV, and assesses their implications for gamma-ray excess signals.

Contribution

The paper provides a detailed analysis of Earth Limb gamma-ray spectra, identifying potential systematic effects and clarifying the nature of observed spectral features near 130 GeV.

Findings

Earth Limb spectrum follows a power-law with index 2.87, consistent with cosmic ray measurements.

Spectral features at 130 GeV in small data subsets are likely statistical fluctuations.

Systematic differences at 2σ level are observed between small and large incidence angle data.

Abstract

We look for possible spectral features and systematic effects in Fermi-LAT publicly available high-energy gamma-ray data by studying photons from the Galactic centre, nearby galaxy clusters, nearby brightest galaxies, AGNs, unassociated sources, hydrogen clouds and Earth Limb. Apart from already known 130 GeV gamma-ray excesses from the first two sources, we find no new statistically significant signal from others. Much of our effort goes to studying Earth Limb photons. In the energy range 30 GeV to 200 GeV the Earth Limb gamma-ray spectrum follows power-law with spectral index 2.87\pm 0.04 at 95 % CL, in a good agreement with the PAMELA measurement of cosmic ray proton spectral index between 2.82-2.85, confirming the physical origin of the Limb gamma-rays. In small subsets of Earth Limb data with small photon incidence angle it is possible to obtain spectral features at different energies, including at 130 GeV, but determination of background, thus their significances, has large uncertainties in those cases. We observe systematic 2\sigma level differences in the Earth Limb spectra of gamma-rays with small and large incidence angles. The behaviour of those spectral features as well as background indicates that they are likely statistical fluctuations.