Observations of G320.4–1.2 and Speculations on Supernova Remnant Morphology

The galactic radio source G320.4–1.2 (MSH15–52) consists of several components, the most prominent of which is situated in the north-west quadrant and is associated with the Hα nebula RCW89. Caswell et al. (1981) mapped the source at 1.4 GHz with a resolution of 50″ arc and concluded that it was a single supernova remnant with all components having spectral index α ≈ −0.34. This SNR has become more significant with the recent discovery (Seward and Harnden, 1982) of an X-ray pulsar of period 150 ms at the position (1950) R.A. 15h09m59s.5, Dec. −58°56′57″ near the centre of the remnant and the detection of this pulsar at radio frequencies (Manchester et al., 1982). The pulsar has some similarities to the Crab pulsar in that its period derivative is extremely high and hence its characteristic age low, ∼1570 years, comparable to that of the Crab pulsar. Timing observations (Manchester and Durdin, unpublished) indicate that the pulsar is not a member of a binary system and hence that the pulsed X-ray emission is powered by rotational energy, as in the Crab pulsar.