Investigating the cosmic-ray ionization rate near the supernova remnant IC 443 through H⁺₃ observations

Observational and theoretical evidence suggests that high-energy Galactic cosmic rays are primarily accelerated by supernova remnants. If also true for low-energy cosmic rays, the ionization rate near a supernova remnant should be higher than in the general Galactic interstellar medium (ISM). We have searched for H⁺₃ absorption features in six sight lines which pass through molecular material near IC 443-a well-studied case of a supernova remnant interacting with its surrounding molecular material-for the purpose of inferring the cosmic-ray ionization rate in the region. In two of the sight lines (toward ALS 8828 and HD 254577) we find large H⁺₃ column densities, N(H⁺₃) ≈ 3 × 10¹⁴ cm^-2, and deduce ionization rates of ζ₂ ≈ 2 × 10^-15 s^-1, about five times larger than inferred toward average diffuse molecular cloud sight lines. However, the 3δ upper limits found for the other four sight lines are consistent with typical Galactic values. This wide range of ionization rates is likely the result of particle acceleration and propagation effects, which predict that the cosmic-ray spectrum and thus ionization rate should vary in and around the remnant. While we cannot determine if the H⁺₃ absorption arises in post-shock (interior) or pre-shock (exterior) gas, the large inferred ionization rates suggest that IC 443 is in fact accelerating a large population of low-energy cosmic rays. Still, it is unclear whether this population can propagate far enough into the ISM to account for the ionization rate inferred in diffuse Galactic sight lines.