Large samples of supernova remnants are needed in order to study the global distribution of supernovae in galaxies, for determining how the environment in which a SN explodes affects the appearance of a SNR, for studying abundances and abundance gradients in galaxies, for estimating SN rates, and in order to determine the energetics of SNRs and their expansion. Here we describe techniques which are currently being used to expand SNR samples in nearby spirals.

Introduction

The Cygnus Loop is thought to be about 18,000 years old (Ku et al. 1984). Assuming a SN rate of 5 per century (van den Bergh & Tammann 1991), there should be about 900 SNR in the galaxy younger than the Cygnus Loop. However, Green's revised catalog of Galactic SNRs contains 182 SNRs, some of which are clearly more evolved than the Cygnus Loop (Green, these proceedings). As a result, it is clear that the Galactic sample is very incomplete. In the Galaxy, nearly all SNRs have been first recognized as SNRs from radio observations. Since SNRs are found primarily in the Galactic plane and since X-rays and optical light are strongly absorbed by material in the Galactic plane, they are hard to detect in these wavelength bands. In fact, only about 40 Galactic SNRs have been detected at optical wavelengths and only about 50 have been detected at X-ray wavelengths.