Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants - Tracing the hadronic origin of GeV gamma radiation

Abstract

Energetic gamma rays (GeV to TeV photon energy) have been detected toward several supernova remnants (SNR) 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 (CR) 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 CR of lower energy (<1 GeV) accompanying the energetic component. These particles of lower energy are most effective in ionizing interstellar gas, leaving an observable imprint on the interstellar ion chemistry. A correlation of energetic gamma radiation with enhanced interstellar ionization can thus support the hadronic origin of the gamma rays and constrain the acceleration of ionizing CR in SNR. We propose a method to test the hadronic origin of GeV gamma rays from SNR associated with a molecular cloud. We use observational gamma ray data for each of these SNR known, modeling the observations to obtain the underlying proton spectrum assuming 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. We show that the 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.

published in Astronomy and Astrophysics, 13 pages, 19 figures

References in corpus (10)

• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• Parameterization of Gamma, e^+/- and Neutrino Spectra Produced by p-p Interaction in Astronomical Environment
• Energy spectra of gamma-rays, electrons and neutrinos produced at interactions of relativistic protons with low energy radiation
• Diffusive Shock Acceleration with Magnetic Amplification by Non-resonant Streaming Instability in SNRs
• Supernova enhanced cosmic ray ionization and induced chemistry in a molecular cloud of W51C
• Exploring a SNR/Molecular Cloud Association Within HESS J1745-303
• Cosmic ray electrons and positrons from supernova explosions of massive stars
• Cosmic Ray streaming from SNRs and gamma ray emission from nearby molecular clouds
• A 7mm line survey of the shocked and disrupted molecular gas towards the W28 field TeV gamma-ray sources
• Parameterization of the Angular Distribution of Gamma Rays Produced by p-p Interaction in Astronomical Environment

Cited by in corpus (11)

• Galactic outflow driven by the active nucleus and the origin of the gamma-ray emission in NGC 1068
• Prediction of the diffuse neutrino flux from cosmic ray interactions near Supernova Remnants
• Extended Carbon Line Emission in the Galaxy: Searching for Dark Molecular Gas along the G328 Sightline
• Spectrum and ionization rate of low energy Galactic cosmic rays
• Interaction between Molecular Clouds and MeV-TeV Cosmic-ray Protons Escaped from Supernova Remnants
• Theoretical study of ionization profiles of molecular clouds near supernova remnants: Tracing the hadronic origin of GeV gamma radiation
• Galactic Propagation of Cosmic Rays from Individual Supernova Remnants
• A statistical classification of the unassociated gamma-ray sources in the second Fermi Large Area Telescope Catalog
• Ionisation as indicator for cosmic ray acceleration
• Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants
• Observations of ionized carbon towards the $\gamma$-ray-bright supernova remnant RX J1713.7-3946