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Published online by Cambridge University Press: 25 May 2016
SN1993J is very unique object which was discovered in the nearby Sb galaxy M81 (NGC 3031) on March 28. The first detection of the radio emission was at 22.5 GHz by the VLA only 5 days after the optical outburst. Subsequently X-ray emission was detected by ROSAT and ASCA at 6 days and 8 days after the explosion respectively. These emissions are expected when the SN shock front sweeps out the circumstellar matter (CSM). The early detection of radio and X-ray emission implies the existence of high-density CSM in the vicinity of the supernova(e.g.).