The Re-Acceleration of the Shock Wave in the Radio Remnant of SN 1987A
  We report on updated radio imaging observations of the radio remnant of
Supernova 1987A (SN 1987A) at 9 GHz, taken with the Australia Telescope Compact
Array (ATCA), covering a 25-year 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 both models show an increasing flux density, and have
shown a continuing expansion of the remnant. As found in previous studies, we
find the torus model most accurately fits our data, and has shown a change in
the remnant expansion at Day 9,300 $\pm$210 from 2,300 $\pm$200 km/s to 3,610
$\pm$240 km/s. 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
$\sim$2020, and note the south-eastern 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 shockwave interacting with the equatorial ring around Day 5,000.
However, we find the emission has become increasingly dominated by the
low-lying regions by Day 9,300, overlapping with the regions of X-ray emission.
We conclude that the shockwave is now leaving the equatorial ring, exiting
first from the south-east region of the remnant, and is re-accelerating as it
begins to interact with the circumstellar medium beyond the dense inner ring.
