Reconsidering the Identification of M101 Hypernova Remnant Candidates

Using a deep Chandra AO-1 observation of the face-on spiral galaxy M101, we examine three of five previously optically identified X-ray sources that are spatially correlated with optical supernova remnants (MF 54, MF 57, and MF 83). The X-ray fluxes from these objects, if due to diffuse emission from the remnants, are bright enough to require a new class of objects, with the possible attribution by Wang to diffuse emission from hypernova remnants. Of the three, MF 83 was considered the most likely candidate for such an object because of its size, nature, and close positional coincidence. However, we find that MF 83 is clearly ruled out as a hypernova remnant by both its temporal variability and spectrum. The bright X-ray sources previously associated with MF 54 and MF 57 are seen by Chandra to be clearly offset from the optical positions of the supernova remnants by several arcseconds, confirming a result suggested by the previous work. MF 54 does have a faint X-ray counterpart, however, with a luminosity and temperature consistent with a normal supernova remnant of its size. The most likely classifications of the sources are as X-ray binaries. Although counting statistics are limited, over the 0.3–5.0 keV spectral band the data are well fitted by simple absorbed power laws with luminosities in the 10³⁸–10³⁹ ergs s^-1 range. The power-law indices are softer than those of Milky Way low-mass X-ray binaries of similar luminosities and are more consistent with those of the Large Magellanic Cloud. Both the high luminosity and the soft spectral shape favor these being accreting black hole binaries in high soft states.