High-resolution radio study of SNR IC 443 at low radio frequencies

Abstract

Aims. 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. 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. 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. 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 (τ 74 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.

A New Look at the Integrated Radio/Microwave Continuum Spectrum of Galactic Supernova Remnant Ic 443

“…The flux density of, already mentioned, pulsar wind nebula in IC 443 at 1.42 GHz is of 0.20 ± 0.04 Jy while the integrated flux density of the SNR is around 140 Jy at the same continuum frequency (Castelletti et al 2011).…”

“…This is in agreement with the model proposed by Rho et al (2001) in which the infrared emission from the ionized species in the east bright radio limb of IC 443 comes from shattered dust produced by a fast dissociating Jtype shock. Assuming an electron temperature in a range between 8000 -12,000 K (consistent with infrared analysis), Castelletti et al (2011) estimated emission measure (EM ) of (2.8 − 5.0) × 10 3 cm −6 pc for the region of the strongest thermal absorption (eastern rim).…”

“…In addition to its apparent dimensions, the rough proximity to the Galactic anti-center location, leaves this remnant relatively well isolated from the confusing effects normally complicating studies of the inner Galactic SNRs (Castelletti et al 2011;Ohnishi et al 2014). A distance of 1.5 kpc (corresponding the diameter of 20 pc) is adopted in most of the papers related to IC 443 although it is still debated (Fesen 1984;Welsh & Sallmen 2003).…”

“…The nature of third, larger, incomplete and faint shell extending beyond the northeast periphery of the remnant is still not clear enough although. Still, it was proposed to be a different SNR (G189.6+3.3), that overlaps with IC 443 (Asaoka & Aschenbach 1994;Leahy 2004;Castelletti et al 2011).…”

“…In order to estimate the initial value of the parameter S ff for the least-square fits (see Section 5), in the case of SNR IC 443, one can use the electron temperatures derived in Castelletti et al (2011) and the value of 10 −4 sr, for the source solid angle Ω S (Green 2014), as S ff = 2k b T e Ω S /c 2 , where k b and c are Boltzmann constant and speed of light, respectively.…”