2001ApJ...547..754L

The supernova remnant (SNR) candidate MF 83 in M101 is coincident with a very luminous X-ray source. Based on the high X-ray luminosity, it has been suggested that MF 83 is a ``hypernova remnant'' requiring an explosion energy about 2 orders of magnitude higher than normal supernovae. We have analyzed high-quality ground-based and Hubble Space Telescope observations of MF 83, and find that MF 83 is a star formation region, consisting of a large ionized gas shell and four H II regions along its periphery. Continuum images show OB associations in these H II regions and within the large shell. The shell has an expansion velocity of ∼50 km.s^–1 and a diameter of ∼270 pc. The optical properties of this shell in MF 83 are similar to those of X-ray-bright superbubbles in the Large Magellanic Cloud. If the X-ray emission is indeed diffuse, the implied thermal energy in MF 83 is high, a few times 10⁵² ergs. This amount of thermal energy requires a large number of concentrated supernova explosions or one powerful explosion. Future X-ray observations with a high angular resolution are needed to resolve the diffuse emission and point sources in MF 83, in order to determine more accurately the thermal energy in the shell interior and its required explosion energy.