The radio evolution of supernova SN2008iz in M82

Abstract

We report on multi-frequency Very Large Array (VLA) and Very Long Baseline Interferometry (VLBI) radio observations for a monitoring campaign of supernova SN2008iz in the nearby irregular galaxy M82. We fit two models to the data, a simple time power-law, S$\propto$ t${}^{\beta }$, and a simplified Weiler model, yielding decline indices, $\beta$ of -1.22$\pm$0.07 (days 100-1500) and -1.41$\pm$0.02 (days 76-2167), respectively. The late-time radio light curve evolution shows flux-density flares at $\sim$970 and $\sim$1400 days which are a factor of $\sim$2 and $\sim$4 higher than the expected flux, respectively. The later flare, besides being brighter, does not show signs of decline at least from results examined so far (2014 January 23, day 2167). We derive the spectral index, $\alpha$, S$\propto$ ${\nu }^{\alpha }$ for frequencies 1.4 to 43GHz for SN2008iz during the period from $\sim$430 to 2167 days after the supernova explosion. The value of $\alpha$ shows no signs of evolution and remains steep $\approx$$-$1 throughout the period, unlike that of SN1993J which started flattening at $\sim$day 970. From the 4.8 and 8.4GHz VLBI images, the supernova expansion is seen to start with shell like structure that gets more and more asymmetric, then breaks up in the later epochs with bright structures dominating the southern part of the ring. This structural evolution differs significantly from SN1993J, which remains circularly symmetric over 4000 days after the explosion. The VLBI 4.8 and 8.4GHz images are used to derive a deceleration index, $m$, for SN2008iz, of 0.86$\pm$0.02, and the average expansion velocity between days 73 and 1400 as (12.1$\pm$0.2)$\times$10${}^3$kms${}^{-1}$.

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