The Reacceleration of the Shock Wave in the Radio Remnant of SN 1987A

We report on updated radio imaging observations of the radio remnant of SN 1987A at 9 GHz, taken with the Australia Telescope Compact Array (ATCA), covering a 25 yr period (1992-2017). We use Fourier modeling of the supernova remnant to model its morphology, using both a torus model and a ring model, and find that both models show an increasing flux density and have shown a continuing expansion of the remnant. As found in previous studies, we find that the torus model most accurately fits our data and has shown a change in the remnant expansion at day 9300 ± 210 from 2300 ± 200 km s^-1 to 3610 ± 240 km s^-1. We have also seen an increase in brightness in the western lobe of the remnant, although the eastern lobe is still the dominant source of emission, unlike what has been observed at contemporary optical and X-ray wavelengths. We expect to observe a reversal in this asymmetry by the year ∼2020, and we note that the southeastern side of the remnant is now beginning to fade, as has also been seen in optical and X-ray data. Our data indicate that high-latitude emission has been present in the remnant from the earliest stages of the shock wave interacting with the equatorial ring around day 5000. However, we find that the emission has become increasingly dominated by the low-lying regions by day 9300, overlapping with the regions of X-ray emission. We conclude that the shock wave is now leaving the equatorial ring, exiting first from the southeast region of the remnant, and is reaccelerating as it begins to interact with the circumstellar medium beyond the dense inner ring.