A New Optical Survey of Supernova Remnant Candidates in M31
  We present a survey of optically emitting supernova remnants (SNRs) in M31 based on H$\alpha$ and [SII] images in the Local Group Survey. Using these images, we select objects that have [SII]:H$\alpha$ $>$ 0.4 and circular shapes. We find 76 new SNR candidates. We also inspect 234 SNR candidates presented in previous studies, finding that only 80 of them are SNR candidates according to our criteria. Combining them with the new candidates, we produce a master catalog of 156 SNR candidates in M31. We classify these SNR candidates according to two criteria: the SNR progenitor type [Type Ia and core-collapse (CC) SNRs] and the morphological type. Type Ia and CC SNR candidates make up 23% and 77%, respectively, of the total sample. Most of the CC SNR candidates are concentrated in the spiral arms, while the Type Ia SNR candidates are rather distributed over the entire galaxy, including the inner region. The CC SNR candidates are brighter in H$\alpha$ and [SII] than the Type Ia SNR candidates. We derive a cumulative size distribution of the SNR candidates, finding that the distribution of the candidates with 17 $< D <$ 50 pc is fitted well by a power law with the power law index $\alpha = 2.53\pm0.04$. This indicates that most of the SNR candidates identified in this study appear to be in the Sedov-Taylor phase. The [SII]:H$\alpha$ distribution of the SNR candidates is bimodal, with peaks at [SII]:H$\alpha$ $\sim$ 0.4 and $\sim$ 0.9. The properties of these SNR candidates vary little with the galactocentric distance. The H$\alpha$ and [SII] surface brightnesses show a good correlation with the X-ray luminosity of the SNR candidates that are center-bright. The SNR candidates with X-ray counterparts have higher surface brightnesses in H$\alpha$ and [SII] and smaller sizes than those without such counterparts. 
Jong Hwan Lee, Myung Gyoon Lee
astro-ph.GA astro-ph.CO