Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants

TL;DR

This paper explores how gamma-ray observations of supernova remnants can be used to infer the acceleration and ionization effects of cosmic rays on nearby molecular clouds, supporting hadronic origin theories.

Contribution

It introduces a method to model cosmic ray spectra from gamma-ray data and extrapolate to lower energies to estimate ionization rates in molecular clouds.

Findings

Gamma-ray data can be used to infer cosmic ray spectra.

Extrapolated spectra help estimate ionization rates in molecular clouds.

Supports hadronic origin of gamma rays in SNRs.

Abstract

Energetic gamma rays (GeV to TeV photon energy) have been detected toward several supernova remnants (SNR) that are associated with molecular clouds. If the gamma rays are produced mainly by hadronic processes rather than leptonic processes like bremsstrahlung, then the flux of energetic cosmic ray nuclei (>1 GeV) required to produce the gamma rays can be inferred at the site where the particles are accelerated in SNR shocks. It is of great interest to understand the acceleration of the cosmic rays of lower energy (<1 GeV) that accompany the energetic component. These particles of lower energy are most effective in ionizing interstellar gas, which leaves an observable imprint on the interstellar ion chemistry. A correlation of energetic gamma radiation with enhanced interstellar ionization can thus be used to support the hadronic origin of the gamma rays and to constrain the acceleration of ionizing cosmic rays in SNR. Using observational gamma ray data, the primary cosmic ray proton spectrum can be modeled for E >1 GeV, and careful extrapolation of the spectrum to lower energies offers a method to calculate the ionization rate of the molecular cloud.