2011A&A...534A..21C

We investigate the morphology at low radio frequencies of the supernova remnant (SNR) IC 443 in detail and accurately establish its radio continuum spectral properties. We used the VLA in multiple configurations to produce high-resolution radio images of IC 443 at 74 and 330MHz. From these data we produced the first sensitive, spatially resolved spectral analysis of the radio emission at long wavelengths. The changes with position in the radio spectral index were correlated with data in near infrared (NIR) from 2MASS, in gamma-rays from VERITAS, and with the molecular ¹²CO (J=1-0) line emission. The new image at 74MHz has HPBW=35'' and rms=30mJy/beam and at 330MHz HPBW=17'' and rms=1.7mJy/beam. The integrated flux densities for the whole SNR are S_74MHz^SNR=470±51Jy and S_330MHz^SNR=248±15Jy. Improved estimates of the integrated spectrum were derived taking a turnover into account to fit the lowest frequency measurements in the literature. Combining our measurements with existing data, we derive an integrated spectral index α_10MHz^10700MHz=-0.39±0.01 with a free-free continuum optical depth at 330MHz τ₃₃₀∼7x10^–4 (τ₁₀=1.07); all measurements above ∼10MHz are equally consistent with a power law spectrum. For the pulsar wind nebula associated with the compact source CXOU J061705.3+222127, we calculated S_330MHz^PWN=0.23±0.05Jy, S_1420MHz^PWN=0.20±0.04Jy, and α_330MHz^8460MHz∼0.0. Substantial variations are observed in spectral index between 74 and 330MHz across IC 443. The flattest spectral components (-0.25≤α≤-0.05) coincide with the brightest parts of the SNR along the eastern border, with an impressive agreement with ionic lines as observed in the 2MASS J and H bands. The diffuse interior of IC 443 has a spectrum steeper than found anywhere in the SNR (-0.85≤α≤-0.6), while the southern ridge again has a flatter spectrum (α~-0.4). With the available statistics the VERITAS γ-ray emission strikingly matches the CO distribution, but no clear evidence is found for a morphological correlation between the TeV distribution and radio emission. The excellent correspondence between the eastern radio flattest spectrum region and NIR ionic lines strongly suggests that the passage of a fast, dissociating J-type shock across the interacting molecular cloud dissociated the molecules and ionized the gas. We therefore conclude that thermal absorption at 74MHz (τ₇₄ up to ∼0.3) is responsible for the localized spectral index flattening observed along the eastern border of IC 443. Towards the interior of IC 443, the spectrum is consistent with those expected from linear diffusive shock acceleration, while the flatter spectrum in the southern ridge is a consequence of the strong shock/molecular cloud interaction.