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The nearby starburst galaxy NGC 253 has been observed at 6, 3.6, 2, and 1.3 cm using the highest resolution configuration of the VLA. Maps with resolutions ranging from $0.1''$ to $0.6''$ are presented. Over a total time span of four years, no new compact radio sources have appeared at 6 cm, and there is no evidence for any sources fading. In M82, Kronberg \& Sramek (1985, Science, 227, 28; 1992, in ``X-ray Emission from AGN and the Cosmic X-ray Background,'' p. 247) have observed no new sources over 10 years and have claimed a general fading of their sources in the intervals 1981--1983 and 1981--1991. However, their published data show that only the brightest source has faded between 1983 and 1991, while the other source flux densities appear to have remained constant over that 7.5-yr interval. Therefore, we believe the source variability results for NGC 253 and M82 are consistent with each other.
The higher frequency NGC 253 data, at resolutions as high as $0.1''$, together with scaled-array spectral-index maps at $0.4''$ to $0.6''$ resolution, reveal at least two different types of sources in the central kiloparsec of NGC 253. Those along the main ridge line tend to have flat spectra and show up even in the highest resolution 2-cm and 1.3-cm images. The sources on either side of that line tend to have steeper spectra and vanish at the highest frequencies. The implication is that at least some of the sources are not young supernovae. Based on this fact and the apparently minimal source variability in NGC 253 and M82, the supernova rates in the two galaxies probably are no greater than $\sim 0.1$/yr.
The research described above was supported by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and by NSF grant AST-9120053 to UCSB.
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