2007AJ....133.1361P

We present a search for X-ray counterparts to optically identified and candidate radio supernova remnants (SNRs) in five nearby galaxies, M81, M101, NGC 2403, NGC 4736 (M94), and NGC 6946, using observations made with the Chandra X-Ray Observatory. A total of 138 optically identified SNRs and 50 candidate radio SNRs in these galaxies were sampled by these observations. Nine optically identified SNRs and 12 candidate radio SNRs are positionally coincident with Chandra-detected X-ray sources that were not already known to be time-variable or associated with X-ray binaries. We used survival statistics to determine if the properties of the optically identified SNRs with and without Chandra-detected counterparts (referred to as group A and group NotA, respectively), as well as the candidate radio SNRs with and without Chandra-detected counterparts (referred to as group B and group NotB, respectively) differ in a statistically significant manner. We find that for the SNRs in groups A and NotA, only the mean value of the diameter d differs significantly between the two groups (26±4 pc compared to 62±6 pc). In addition, for the SNRs in groups B and NotB, we find that only the spectral index α differs significantly between the two groups (0.6±0.1 compared to 0.9±0.1). We find no correlation between unabsorbed X-ray and optical luminosities for the group A SNRs and no correlation between unabsorbed X-ray and radio luminosities for the group B SNRs: this result indicates that the interstellar medium surrounding these SNRs is inhomogeneous rather than uniform. We claim that the higher incidence of Chandra-detected counterparts for candidate radio SNRs compared to the optically identified SNRs (as noticed in previous works) illustrates the role played by ambient density in affecting searches for SNRs in nearby galaxies at multiple wavelengths. We argue that deep systematic X-ray, optical, and radio observations of other galaxies are necessary to examine the multiwavelength properties of SNRs, to explore wavelength-dependent selection effects in more detail, and to search for time variability in the emission from X-ray counterparts to optically identified SNRs and candidate radio SNRs.