Evidence of a Spectral Break in the Gamma-Ray Emission of the Disk Component of the Large Magellanic Cloud: A Hadronic Origin?

Abstract

It has been suggested that the high-energy gamma-ray emission (> 100 MeV) of nearby star-forming galaxies may be produced predominantly by cosmic rays colliding with the interstellar medium through neutral pion decay. The pion decay mechanism predicts a unique spectral signature in the gamma-ray spectrum, characterized by a fast-rising spectrum (in (EF)-F-2(E) representation) and a spectral break below a few hundred MeV. Here, we report evidence of a spectral break, around 500. MeV in the disk emission of the Large Magellanic Cloud (LMC), that was found during an analysis of the gamma-ray data extending down to 60 MeV, observed by the Fermi-Large Area Telescope. The break is consistent with the pion decay model of gamma-ray emission, although leptonic models, such as electron bremsstrahlung emission, cannot be ruled out completely