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Detection of X-Ray-emitting Hypernova Remnants in M101

Based on an ultradeep (230 ks) ROSAT HRI imaging of M101, we have detected five X-ray sources that coincide spatially with optical emission line features previously classified as supernova remnants (SNRs) in this nearby galaxy. Two of these coincidences (SNR MF 83 and NGC 5471B) most likely represent the true physical association of X-ray emission with shock-heated interstellar gas. MF 83, with a radius of ~134 pc, is one of the largest remnants known. NGC 5471B, with a radius of 30 pc and a velocity of at least 350 km s-1 (FWZI), is extremely bright in both radio and optical. The X-ray luminosities of these two shell-like remnants are ~1 and 3×1038 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 blast-wave energy is ~3×1052 ergs for NGC 5471B and ~3×1053 ergs for MF 83. Therefore, the remnants likely originate in hypernovae, which are a factor of ≳10 more energetic than canonical supernovae and are postulated as being responsible for gamma-ray bursts observed at cosmological distances. The study of such hypernova remnants in nearby galaxies has the potential to provide important constraints on the progenitor type, rate, energetics, and beaming effect of gamma-ray bursts.