Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants. Tracing the hadronic origin of GeV gamma radiation
Artikel i vetenskaplig tidskrift, 2012

Context. Energetic gamma rays (GeV to TeV photon energy) have been detected toward several supernova remnants (SNRs) 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. Aims. We propose a method to test the hadronic origin of GeV gamma rays from SNRs associated with a molecular cloud. Methods. We use observational gamma ray data for each SNR known to be associated with a molecular cloud, modeling the observations to obtain the underlying proton spectrum under the assumption that the gamma rays are produced by pion decay. Assuming that the acceleration mechanism does not only produce high energy protons, but also low energy protons, this proton spectrum at the source is then used to calculate the ionization rate of the molecular cloud. Ionized molecular hydrogen triggers a chemical network forming molecular ions. The relaxation of these ions results in characteristic line emission, which can be predicted. Results. We show that the predicted ionization rate for at least two objects is more than an order of magnitude above Galactic average for molecular clouds, hinting at an enhanced formation rate of molecular ions. There will be interesting opportunities to measure crucial molecular ions in the infrared and submillimeter-wave parts of the spectrum.

cosmic rays


diffusive shock acceleration

ic 443

astronomical environments

p-p interaction

radiation mechanisms: non-thermal


ISM: clouds

astroparticle physics


astrophysical shocks

large-area telescope

ISM: supernova

cold media


F. Schuppan

Ruhr-Universität Bochum

Julia K. Becker

Ruhr-Universität Bochum

John H Black

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

S. Casanova

Université Paris Diderot

North-West University

Ruhr-Universität Bochum


Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 541 A126 (pp. 1-10) A126


Astronomi, astrofysik och kosmologi


Grundläggande vetenskaper


Onsala rymdobservatorium



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