High-resolution radio study of SNR IC443 at low radio frequencies

Abstract: We investigated in detail the morphology at low radio frequencies of the supernova remnant IC443 and accurately established its radio continuum spectral properties. We used the VLA in multiple configurations to produce high resolution radio images of IC443 at 74 and 330 MHz. The changes with position in the radio spectral index were correlated with data in near infrared from 2MASS, in gamma-rays from VERITAS, and with the molecular 12^CO line emission. The new image at 74 MHz has HPBW=35", rms=30 mJy/beam and at 330 MHz HPBW= 17" and rms=1.7 mJy/beam. The integrated flux densities for the whole SNR are S_74MHz=470+/-51 Jy and S_330MHz=248+/-15 Jy. For the pulsar wind nebula associated with the compact source CXOUJ061705.3+222127, we calculated S_330MHz=0.23+/-0.05 Jy, S_1420MHz=0.20+/-0.04 Jy, and alpha~0.0. Substantial variations are observed in spectral index between 74 and 330 MHz across IC443. The flattest spectral components (-0.25< alpha<-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. This result 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 is responsible for the localized spectral index flattening observed along the eastern border of IC443. The diffuse interior of IC443 has a spectrum steeper than found anywhere in the SNR (-0.85<alpha<-0.6), while the southern ridge again has a flatter spectrum (alpha -0.4), consequence in this case of the strong shock/molecular cloud interaction. 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.