Fourier modeling of the radio torus surrounding SN 1987A

We present detailed Fourier modeling of the radio remnant of SN 1987A using high-resolution 9 and 18 GHz data taken with the Australia Telescope Compact Array over the period 1992-2008. We develop a parameterized three-dimensional torus model for the expanding radio shell, in which the emission is confined to an inclined equatorial belt; our model also incorporates both a correction for light-travel time effects and an overall east-west gradient in the radio emissivity. By deriving an analytic expression for the two-dimensional Fourier transform of the projected three-dimensional brightness distribution, we can fit our spatial model directly to the interferometric visibility data. This provides robust estimates of the radio morphology at each epoch. The best-fit results suggest a constant remnant expansion at 4000 +/- 400 km s(-1) over the 16 yr period covered by the observations. The model fits also indicate substantial midlatitude emission, extending to +/- 40 degrees on either side of the equatorial plane. This likely corresponds to the extraplanar structure seen in H alpha and Ly alpha emission from the supernova reverse shock, and broadly supports hydrodynamic models in which the complex circumstellar environment was produced by a progression of interacting winds from the progenitor. Our model quantifies the clear asymmetry seen in the radio images: we find that the eastern half of the radio remnant is consistently similar to 40% brighter than the western half at all epochs, which may result from an asymmetry in the ejecta distribution between these two hemispheres.