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

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.

💡 Research Summary

The paper addresses a long‑standing question in high‑energy astrophysics: whether the GeV–TeV γ‑rays detected from supernova remnants (SNRs) that are interacting with molecular clouds are produced predominantly by hadronic processes (π⁰ decay following proton–proton collisions) rather than by leptonic mechanisms such as bremsstrahlung or inverse‑Compton scattering. The authors propose a novel, indirect test of the hadronic scenario by exploiting the fact that the same population of cosmic‑ray (CR) protons that generates the γ‑rays must also contain a substantial low‑energy component (< 1 GeV). These low‑energy protons are highly efficient at ionizing molecular hydrogen (H₂), thereby initiating a cascade of ion‑molecule chemistry that yields observable molecular ions (e.g., H₃⁺, OH⁺, H₂O⁺). The key steps of the analysis are as follows:

1. γ‑Ray Modeling and Proton Spectrum Reconstruction
   Using archival γ‑ray spectra from instruments such as Fermi‑LAT, H.E.S.S., and MAGIC for a sample of SNR–cloud complexes, the authors fit the data with a π⁰‑decay model. The fit yields a power‑law proton spectrum N(E) ∝ E⁻ᵖ with a normalization that reproduces the observed γ‑ray flux. The spectral index p typically lies between 2.2 and 2.4, consistent with diffusive shock acceleration predictions.

2. Extension to Low Energies and Ionization Rate Calculation
   The reconstructed proton spectrum is extrapolated down to ≈10 MeV, assuming the same power‑law continues without a low‑energy cutoff. The ionization rate ζ in the adjacent molecular cloud is then computed by integrating the product of the proton flux, the energy‑dependent ionization cross‑section σ_ion(E), and the stopping power of the cloud material. The authors adopt up‑to‑date σ_ion(E) data from laboratory measurements and account for energy losses due to ionization, Coulomb scattering, and nuclear interactions.

3. Chemical Network and Observable Ion Signatures
   An ionization rate of ζ ≈ 10⁻¹⁴–10⁻¹³ s⁻¹ (i.e., >10 × the Galactic average of ≈10⁻¹⁶ s⁻¹) dramatically enhances the abundances of primary ions H₂⁺ and H⁺, which rapidly react to form H₃⁺, OH⁺, H₂O⁺, and H₃O⁺. The authors employ a steady‑state astrochemical model to predict column densities and line intensities for these species. In particular, H₃⁺ produces strong near‑infrared absorption lines around 3.5–4 µm, while OH⁺ and H₂O⁺ emit in the far‑infrared/sub‑millimeter regime (≈1–2 THz).

4. Case Studies: W44 and IC 443
   Applying the methodology to two well‑studied SNR–cloud systems, W44 and IC 443, the authors find ζ values of order 10⁻¹³ s⁻¹, i.e., more than an order of magnitude above the Galactic average. This elevated ionization is consistent with the γ‑ray‑derived proton spectra and provides independent evidence that the γ‑rays are of hadronic origin.

5. Observational Prospects and Future Work
   The paper outlines concrete observational strategies: high‑resolution infrared spectroscopy (e.g., with JWST/NIRSpec) to detect H₃⁺ absorption, and sub‑millimeter spectroscopy (e.g., with ALMA or NOEMA) to map OH⁺ and H₂O⁺ emission. Spatial correlation between regions of intense γ‑ray emission and enhanced ion‑molecule line intensities would constitute a compelling validation of the hadronic model. Moreover, the authors suggest that combining multi‑wavelength data with three‑dimensional magneto‑hydrodynamic simulations could eventually provide a unified picture of CR acceleration, propagation, and feedback on the interstellar medium.

In summary, the study demonstrates that low‑energy CR protons, inferred from high‑energy γ‑ray observations, can produce a measurable increase in the ionization rate of nearby molecular clouds. The resulting chemical signatures offer a powerful, complementary diagnostic to directly test the hadronic origin of SNR‑associated γ‑rays and to probe the acceleration of sub‑GeV cosmic rays in shock environments.