On the shape of the gamma-ray spectrum around the "$\pi^0$-bump"

TL;DR

This paper investigates the detailed shape of the gamma-ray spectrum around the pion-decay bump, considering various radiation contributions and their effects on the spectral features, using recent cross-section parametrizations and a time-dependent model.

Contribution

It provides a comprehensive analysis of the gamma-ray spectrum formation around the pion-decay bump, including secondary radiation and heavy ion interactions, with a time-dependent approach.

Findings

Secondary electron bremsstrahlung influences the spectrum below the bump.

Heavy ion interactions contribute significantly below 100 MeV.

The model clarifies the spectral shape around the pion-decay feature.

Abstract

The "pion-decay" bump is a distinct signature of the differential energy spectrum of $\gamma$-rays between 100 MeV and 1 GeV produced in hadronic interactions of accelerated particles (cosmic rays) with the ambient gas. We use the recent parametrisations of relevant cross-sections to study the formation of the "pion-decay" bump. The $\gamma$-ray spectrum below the maximum of this spectral feature can be distorted because of contributions of additional radiation components, in particular, due to the bremsstrahlung of secondary electrons and positrons, the products of decays of $\pi^\pm$-mesons, accompanying the $\pi^0$-production. At energies below 100 MeV, a non-negligible fraction of $\gamma$-ray flux could originate from interactions of sub-relativistic heavy ions. We study the impact of these radiation channels on the formation of the overall $\gamma$-ray spectrum based on a time-dependent treatment of evolution of energy distributions of the primary and secondary particles in the $\gamma$-ray production region.