Context :

Aims : To investigate in detail the morphology at low radio frequencies of the supernova remnant ( SNR ) IC 443 and to accurately establish its radio continuum spectral properties .

Methods : We used the VLA in multiple configurations to produce high resolution radio images of IC 443 at 74 and 330 MHz .
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 ^ { 12 } CO ( J =1–0 ) line emission .

Results : The new image at 74 MHz has HPBW = 35 ^ { \prime \prime } , rms=30 mJy beam ^ { -1 } and at 330 MHz HPBW = 17 ^ { \prime \prime } and rms=1.7 mJy beam ^ { -1 } .
The integrated flux densities for the whole SNR are S _ { \mathrm { 74 MHz } } ^ { \mathrm { SNR } } = 470 \pm 51 Jy and S _ { \mathrm { 330 MHz } } ^ { \mathrm { SNR } } = 248 \pm 15 Jy .
Improved estimates of the integrated spectrum were derived taking into account a turnover to fit the lowest frequency measurements in the literature .
Combining our measurements with existing data , we derive an integrated spectral index \alpha _ { \mathrm { 10 MHz } } ^ { \mathrm { 10700 MHz } } = -0.39 \pm 0.01 with a free-free continuum optical depth at 330 MHz \tau _ { 330 } \sim 7 \times 10 ^ { -4 } ( \tau _ { 10 } = 1.07 ) ; all measurements above \sim 10 MHz 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 _ { \mathrm { 330 MHz } } ^ { \mathrm { PWN } } = 0.23 \pm 0.05 Jy , S _ { \mathrm { 1420 MHz } } ^ { \mathrm { PWN } } = 0.20 \pm 0.04 Jy , and \alpha _ { \mathrm { 330 MHz } } ^ { \mathrm { 8460 MHz } } \sim 0.0 .
Substantial variations are observed in spectral index between 74 and 330 MHz across IC 443 .
The flattest spectral components ( -0.25 \leq \alpha \leq - 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 \leq \alpha \leq - 0.6 ) , while the southern ridge again has a flatter spectrum ( \alpha \sim - 0.4 ) .
At the available statistics the VERITAS \gamma -ray emission strikingly matches the CO distribution , but no clear evidence is found for a morphological correlation between the TeV distribution and radio emission .

Conclusions : 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 74 MHz ( \tau _ { 74 } up to \sim 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 .