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Date: Mon, 12 Apr 93 16:12:49 EDT From: Kurt w. Weiler <kweiler@shimmer.nrl.navy.mil> Subject: Radio Supernova SN1993J Update SN 1993J in NGC 3031 S. D. Van Dyk and K. W. Weiler, Naval Research Laboratory; M. P. Rupen and R. A. Sramek, National Radio Astronomy Observatory; and N. Panagia, Space Telescope Science Institute, report that new observations of SN 1993J made on 1993 April 8.33 UT with the Very Large Array in B configuration show that the source is now detectable in at least three frequency bands. This confirms the first weak detection (IAUC 5752) of the supernova at 1.3-cm wavelength (22.5 GHz) on April 2.30 UT. The source has now risen dramatically in flux density at 1.3-cm to 18.94 +/- 0.20 mJy at position R.A. = 09h51m19s.042, Decl. = +69 15' 26".45 (equinox 1950.0 ; +/- 0".2 in each coordinate), in excellent agreement with the previously reported position. The supernova is also detected at 2.0-cm wavelength (14.9 GHz) with flux density 8.04 +/- 0.18 mJy, confirming the previous detection made at this wavelength by Pooley & Green (IAUC 5751). At 3.6-cm wavelength (8.4 GHz) a 0.74 +/- 0.05 mJy source is detected. The radio spectrum of the supernova is optically thick as is expected at this time. The radio position for the supernova compared with the radio position for the nucleus of NGC 3031 given in IAUC 5752, yields an offset for the supernova of 44".0 W, 161".8 S. Comparison with the Type II-Linear radio supernovae SN 1979C and SN 1980K shows that SN 1993J has turned on in the radio significantly earlier than the two previous supernovae. Adopting an explosion date of 25 March 1993, this implies that there is less absorption external to the radio emitting source for SN 1993J and therefore, from modelling, that the progenitor of SN 1993J was probably less massive than the progenitors of the other two radio supernovae. This is assuming that SN 1993J is Type II-Linear as were SN 1979C and SN 1980K; if the supernova is Type II-Plateau, then its behavior in the radio is unpredictable, since a supernova with this subtype has never been previously detected at radio wavelengths. Radio monitoring is continuing on a nearly daily basis at these and other wavelengths in order to follow the supernova's rapidly changing behavior.
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