In the recent past supernova remnants (SNR) have been identified in external galaxies as far as 5 Mpc. These galaxies are: M 33(12), M 31(12), NGC 6822(1), IC 1613(1), NGC 2403(2), NGC 300 (2), NGC 4449(1), where the numbers of SNR confirmed by optical spectroscopy are given in parenthesis. Data for objects in the Magellanic Clouds are discussed extensively elsewhere in this volume.

There are at least 25 supernova remnants (SNR) in the Large Magellanic Cloud (LMC) with X-ray luminosities exceeding 2 × 1035 erg s−1. As many as 25 other SNR may be contained in the X-ray survey conducted with the Einstein Observatory of the LMC. The X-ray spectra of the 6 SNR observed with the Solid State Spectrometer (SSS) resemble their galactic counterparts; two SNR, N157B and 0540–69.3, may emit X-rays primarily by synchrotron radiation. The density of the medium in which SNR are expanding inferred from the X-ray data appears to decrease with SNR diameter; the density of the ISM inferred from the Balmer lines of 4 new SNR in the LMC is much lower than that inferred from X-ray observations. The apparent thermal energy content of LMC SNR evolves with diameter, peaking at ∼5 × 1050 ergs. The ratio of the densities of the X-ray and [SII] emitting plasmas is consistent with their being in pressure equilibrium. The SN rate in the LMC is ∼1 per 100–200 years. This is the number of SN expected from other considerations. The number diameter relation of LMC SNR is consistent with free expansion. The X-ray data are difficult to understand in terms of traditional Sedov models on SNR evolution; probably ejecta and multiphase ISM are required to explain the X-ray properties of LMC SNR.

The Small and Large Magellanic Clouds (SMC and LMC) are the nearest neighbouring galaxies. Their proximity enables us to investigate these galaxies in X-rays in fair detail.

Optical identifications of 32 X-ray sources in the Magellanic Clouds confirm that they are SNRs. They are separated into four classes: the evolved, the oxygen-rich, the Balmer-dominated and the Crab-like. High velocity HI emission is observed from an extended region near 0525–66.0. It is suggested that this is produced by a possible Type III supernova which occurred out of the plane of the LMC and on the far side of the disk. The cumulative number-diameter relation for the LMC SNRs shows that they have evolved much faster than expected from the Sedov theory. It is suggested that this apparent “free-expansion” up to quite large diameters is due to the gradual conversion of the kinetic energy of the ejecta into thermal energy as they overtake the decelerating blast wave.

An understanding of the radio properties and evolution of Galactic supernova remnants has always been hampered by the difficulty of measuring distances. A conventional wisdom has developed around a set of ‘good’ calibrators but most workers involved have drawn attention to the uncertainties and the possibility of selection effects distorting results. This major difficulty is completely overcome by studying SNRs in the Magellanic Clouds. Although there is uncertainty in the absolute distance scale, relative distances can be determined to better than 10% and differences of this magnitude are not significant when intercomparing SNRs. There is, however, another set of problems associated with sensitivity and resolution. The Clouds are an order of magnitude more distant than the average distance of Galactic SNRs, thus many of the SNRs are close to or below the sensitivity limits of most of the southern radiotelescopes and, until recently, the resolution available has often been inadequate to separate non-thermal sources from thermal HII regions, so that both flux densities and spectra have been subject to error. Also there are ∼ 1000 extragalactic background sources which can mimic the flux density and spectra of SNRs in the Clouds, particularly when close to or behind HII regions; as a result numerous incorrect or doubtful SNR identifications have been suggested.

We report the optical identification of four Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud. Both the Balmer-dominated spectra and the presence of a broad Hα component in one remnant can be understood in terms of a very high velocity non-radiative shock encountering gas which is partially neutral, as proposed originally by Chevalier and Raymond to account for the similar spectra of the galactic remnants, Tycho and SN1006. From a consideration of the optical and X-ray luminosities of the SNR with broad Hα emission, we infer that the fraction of neutral gas in the medium is ≲ 30%. Radio observations of the LMC remnants show that their surface brightnesses are anomalously low; this could be intrinsic to the supernova themselves, or a result of their environment. Finally, we argue that the four SNRs all resulted from Type I supernovae, in which case they are the first such remnants to be identified outside the Galaxy.

The work we have performed on some of the SNRs and ring-shaped nebulae of the LMC refers mainly to: photographic plate Hα and [SII] (λλ 6717 Å) imagery and Fabry-Perot (FP) interferometry.

The error box of the unusual Gamma-Ray Burst of March 5, 1979 falls completely inside the optical and radio image of the Supernova Remnant N49 in the Large Magellanic Cloud. This region was observed twice in x-rays with the High Resolution Imager of the Einstein Observatory, six weeks and nearly two years after the Gamma-Ray Burst. We show the comparison between the two observations.

A powerful young supernova remnant (SNR) similar to Cas A has recently been discovered in the irregular galaxy NGC 4449. We have obtained X-ray, optical and ultraviolet data which allow us to investigate possible models for this object and estimate its age. Several lines of argument indicate a massive star of order 25 M⊙ as the precursor to this remnant. If the x-ray emission is attributed to a reverse shock in the ejecta, the remnant should be ∼ 120 years old.

Large numbers (∼30) of supernova remnant candidates have been revealed in new VLA maps of the inner 600 pc of M82 with subarcsecond resolution. The flux density decrease of the bright source 41.9+58 in the M82 nuclear region is found to be occurring with an approximately constant spectral index of −0.9 at ν≳3 GHz, and the low frequency turnover has moved from ∼700 MHz in 1974–1975 to <400 MHz in 1979–81.

A model of 41.9+58 is proposed in which it is, at least initially confined by its own hot (∼108K) pre-SN ejecta, and its relativistic particles are accelerated by a pulsar at its centre.

Clumpy irregular galaxies contain 5–10 “clumps” which are hyperactive HII complexes each equivalent to 100 giant HII regions of the 30 Doradus type (Heidmann 1982). We observed one of them, Mkn 325 (= NGC 7673), with the Einstein IPC in Dec. 1980 (seq.no 10201) for 3,200 s. The reduction was made kindly by D.E. Harris. The source was localized at 23h 25m 12.2s, + 23° 18′ 25″ (1950) in agreement with the optical position, at a quite weak level (14 counts in the 1.4–2.9 kev range). Thus we could not get valuable spectral information, only that the spectrum is rather not soft. Correction for galactic absorption NH = 5 ×1020 at.cm−2 (Heyles 1975) is applied. Fits of power law spectra happen to all go through the point with flux density 4.5 ×10−5 mJy at 3.0 ×1017 Hz (1.24 kev) and they yield a flux (1.1 ± 0.3) x10−13 erg cm−2 s−1 inside 1–3 kev and an X-luminosity (2.2 ± 0.3)×1041 erg s−1 inside 0.5–4.5 kev, for a distance 49 Mpc.

At this conference results have been presented on a number of individual galactic supernova remnants, but many others remain unstudied. It therefore seemed worthwhile to present a catalogue of all presently known SNR's in the Galaxy. Objects of which the true nature is not yet well established have, as far as possible, been omitted. Remnants which have been detected at optical wavelengths are marked by an asterisk in Table 1. Data on the optical identifications are from van den Bergh (1978), supplemented by recent results of Zealey, Elliot and Malin (1979), Reich, Kallas and Steube (1979), Downes, Pauls and Salter (1980), van den Bergh (1981) and Reich and Braunsfurth (1981). Also marked in the table are supernova remnants that have been detected in x-rays. These x-ray identifications are from miscellaneous sources.