Reconsidering the Identification of M101 Hypernova Remnant Candidates

Abstract

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 which are spatially correlated with optical supernova remnants (MF54, MF57, and MF83). 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, MF83 was considered the most likely candidate for such an object due to its size, nature, and close positional coincidence. However, we find that MF83 is clearly ruled out as a hypernova remnant by both its temporal variability and spectrum. The bright X-ray sources previously associated with MF54 and MF57 are seen by Chandra to be clearly offset from the optical positions of the supernova remnants by several arc seconds, confirming a result suggested by the previous work. MF54 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 fit by simple absorbed power laws with luminosities in the 10^38 to 10^39 ergs/s range. The power law indices are softer than those of Milky Way LMXB 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.