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D. Wang (Northwestern University), Y. Chu (University of Illinois)
Recent observations of afterglows of Gamma-ray bursts strongly suggest that they represent the most energetic explosions we have witnessed in the universe. Although the beaming effect of the afterglow emission is still greatly uncertain, the energy release inferred for such an explosion easily exceeds the canonical supernova energy by orders of magnitude. Similar super-energetic explosions in nearby galaxies can naturally leave distinct relics that may last millions of years. Based on data from ROSAT and HST as well as ground-based optical observations, we have identified in the nearby galaxy M101 two remnants that most likely resulted from super-energetic explosions: MF83, with a radius of ~134~pc, is one of the largest remnants known; NGC5471B, with a radius of 30~pc and a velocity of at least 175{\rm~km~s-1} (HWZI), is extremely bright in both radio and optical. The X-ray luminosities of these two shell-like remnants are ~1 and 3 \times 1038 {\rm~ergs~s-1} (0.5-2~keV), about an order of magnitude brighter than the brightest supernova remnants known in our Galaxy and in the Magellanic Clouds. The inferred blastwave energy is ~3 \times 1052~ergs for NGC5471B and ~3 \times 1053~ergs for MF83. The study of such super-energetic remnants has the potential to provide important constraints on the progenitor type, rate, energetics, and beaming effect of Gamma-ray bursts.
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