Reconsidering the Identification of M101 Hypernova Remnant Candidates

Using a deep Chandra AO-1 observation of the face-on spiral galaxy M101, weexamine three of five previously optically-identified X-ray sources which arespatially correlated with optical supernova remnants (MF54, MF57, and MF83).The X-ray fluxes from these objects, if due to diffuse emission from theremnants, are bright enough to require a new class of objects, with thepossible attribution by Wang to diffuse emission from hypernova remnants. Ofthe three, MF83 was considered the most likely candidate for such an object dueto its size, nature, and close positional coincidence. However, we find thatMF83 is clearly ruled out as a hypernova remnant by both its temporalvariability and spectrum. The bright X-ray sources previously associated withMF54 and MF57 are seen by Chandra to be clearly offset from the opticalpositions of the supernova remnants by several arc seconds, confirming a resultsuggested by the previous work. MF54 does have a faint X-ray counterpart,however, with a luminosity and temperature consistent with a normal supernovaremnant of its size. The most likely classifications of the sources are asX-ray binaries. Although counting statistics are limited, over the 0.3--5.0 keVspectral band the data are well fit by simple absorbed power laws withluminosities in the 10^38 to 10^39 ergs/s range. The power law indices aresofter than those of Milky Way LMXB of similar luminosities, and are moreconsistent with those of the Large Magellanic Cloud. Both the high luminosityand the soft spectral shape favor these being accreting black hole binaries inhigh soft states.Comment: 10 pages, 4 figures, ApJ Accepte