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SN~1993J in M~81 (NGC~3031) is the brightest supernova to have occurred in the northern hemisphere in almost 60 years. The last supernova this bright which could easily be seen from northern latitudes was in 1937 (SN~1937C in the galaxy IC~4182) with the other two bright supernovae, SN~1972E and SN~1987A, either difficult or impossible to view from the North. Thus, it is no surprise that since its discovery by the amateur astronomer Francisco Garcia Diez of Lugo, Spain on the night of 1993 March 28, SN~1993J has become one of the best studied supernovae in history, even rivaling the spectacular SN~1987A.
Since its discovery with a small, 10-inch telescope, the most powerful ground-based telescopes and satellites available to modern astronomy have been turned to study SN~1993J and have detected it at all wavelengths from centimeter radio, through IR and optical, to X-ray and $\gamma$-ray. Since SN~1993J is a ``more normal'' SN than SN~1987A, it may contribute even more to our understanding of the general classes of SNe.
In its early optical spectra, SN~1993J showed the clear lines of hydrogen emission required for Type II identification but quickly began to exhibit strong He I lines and a relative decrease of the H$\alpha$ line. This has led some workers to suggest that SN~1993J may be a stripped supergiant star which lost most of its extended hydrogen envelope either through a dense presupernova stellar wind or mass transfer to a close binary companion. The optical light curve has shown an unusual double peak with the first peak interpreted as due to shock breakout and the second driven by radioactive decay.
X-ray emission was detected from SN~1993J only 6 days after optical discovery and debate continues as to whether it is of thermal or non-thermal origin. It is evidence, in any case, for shock interaction with a dense circumstellar medium.
Radio observations have produced more information at more wavelengths than for any previous supernova, with the first radio detection at 1.3 cm only 5 days after optical discovery. SN~1993J has subsequently been detected at wavelengths ranging from 20 cm to 3 mm and is still being monitored at 20, 6, 3.6, 2, and 1.3 cm. The radio emission describes the density and structure of the circumstellar material and its interaction with the SN shock. It also permits an estimate of the properties of the progenitor stellar system before explosion.
Regular VLBI observations are underway at several frequencies and measurements of the angular rate of expansion and radio structure have been obtained. Never before, in any wavelength range, has it been possible to watch the development of supernova structure on milliarcsecond scales.
SN~1993J offers a unique opportunity for study at many wavelengths. Its observation and interpretation will greatly advance our understanding of this energetic and spectacular phenomenon.
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