Detection of the Characteristic Pion-Decay Signature in Supernova Remnants

TLDR

Cosmic rays are mainly relativistic protons, and although supernova remnants are widely accepted as their sources, clear evidence of proton acceleration in SNRs remains elusive. The study aims to detect proton acceleration sites in SNRs by identifying the pion‑decay gamma‑ray signature. The authors use gamma‑ray spectroscopy to distinguish pion‑decay photons from electron‑produced gamma rays, exploiting the characteristic spectral feature produced when accelerated protons collide with interstellar matter. The Fermi Large Area Telescope detected the pion‑decay signature in IC 443 and W44, confirming that cosmic‑ray protons are accelerated in supernova remnants.

Abstract

Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.