2009CBET.1803....1B

A. Brunthaler, K. M. Menten, and C. Henkel, Max-Planck-Institut fuer Radioastronomie; M. J. Reid, Harvard-Smithsonian Center for Astrophysics; G. C. Bower, University of California, Berkeley; and H. Falcke, University of Nijmegen and ASTRON, report the discovery of a radio supernova in M82. The position of 2008iz is R.A. = 9:55:51.55, Decl. = +69:40:45.8 (equinox 2000.0), which is 2".0 west and 1".3 south of the radio kinematic center (Weliachew et al. 1984, A.Ap. 137, 335). SN 2008iz was first discovered as a bright radio transient of unknown origin (cf. http://www.astronomerstelegram. org/?read=2020; also, Brunthaler et al. 2009, A.Ap., in press, with a pre- publication version posted at http://lanl.arxiv.org/abs/0904.2388). VLBI images taken with the NRAO Very Long Baseline Array, the phased Very Large Array, the Green Bank Telescope, and the Effelsberg 100-m telescope on 2008 May 3 and 2009 Apr. 8 at 22 GHz show (at sub-milliarcsecond resolution) a ring-like structure with an expansion velocity of roughly 11000 km/s (for an assumed distance of 3.6 Mpc). This confirms the supernova nature of this source. The measured expansion rate indicates an explosion date in late January 2008. Very Large Array observations on 2009 Apr. 27 at 1.4, 4.8, 8.4, 22, and 43 GHz indicate an optically thin syncrotron sprectrum with a spectral index of -0.8 (typical for type-II supernovae).