Investigating the Cosmic-Ray Ionization Rate Near the Supernova Remnant IC 443 Through H3+ Observations

TL;DR

This study investigates cosmic-ray ionization near supernova remnant IC 443 by observing H3+ absorption, finding elevated ionization rates in some sight lines indicating local cosmic-ray acceleration, but with variability across different regions.

Contribution

It provides observational evidence of increased low-energy cosmic-ray ionization rates near IC 443, highlighting spatial variability and the potential for supernova remnants to accelerate cosmic rays affecting the surrounding ISM.

Findings

Elevated ionization rates (~2×10^-15 s^-1) detected in some sight lines near IC 443.

Ionization rates vary widely, with some consistent with typical Galactic values.

Evidence suggests IC 443 accelerates low-energy cosmic rays impacting local molecular clouds.

Abstract

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 H3+ absorption features in 6 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 2 of the sight lines (toward ALS 8828 and HD 254577) we find large H3+ column densities, N(H3+)~3*10^14 cm^-2, and deduce ionization rates of zeta_2~2*10^-15 s^-1, about 5 times larger than inferred toward average diffuse molecular cloud sight lines. However, the 3 sigma upper limits found for the other 4 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 H3+ 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.