Detection of X-Ray–emitting Hypernova Remnants in M101

Based on an ultra deep (230 ks) ROSAT HRI imaging of M101, we have detected 5X-ray sources that coincide spatially with optical emission line featurespreviously classified as supernova remnants in this nearby galaxy. Two of thesecoincidences (SNR MF83 and NGC5471B) most likely represent the true physicalassociation of X-ray emission with shock-heated interstellar gas. MF83, with aradius of ~ 134 pc, is one of the largest remnants known. NGC5471B, with aradius of 30 pc and a velocity of at least 350 km/s (FWZI), is extremely brightin both radio and optical. The X-ray luminosities of these two shell-likeremnants are $\sim 1$ and $3 \times 10^{38} ergs/s$ (0.5-2 keV), about an orderof magnitude brighter than the brightest supernova remnants known in our Galaxyand in the Magellanic Clouds. The inferred blastwave energy is $\sim 3 \times10^{52} ergs$ for NGC5471B and $\sim 3 \times 10^{53}$ ergs for MF83.Therefore, the remnants likely originate in hypernovae, which are a factor of$\gtrsim 10$ more energetic than canonical supernovae and are postulated asbeing responsible for Gamma-ray bursts observed at cosmological distances. Thestudy of such hypernova remnants in nearby galaxies has the potential toprovide important constraints on the progenitor type, rate, energetics, andbeaming effect of Gamma-ray bursts.Comment: 10 pages, 2 gif figures, Accepted for publication in Astrophysical Journal Letter