The Asymmetric Radio Remnant of SN 1987A

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Abstract
We present seven years of radio observations of SN 1987A made with the Australia Telescope Compact Array. At 1.4, 2.4, 4.8, and 8.6 GHz, the flux density of the radio remnant has increased monotonically since emission was redetected 1200 days after the explosion. On day 3200, the remnant was expanding at 2800 ± 400 km s-1, which we interpret as indicating significant deceleration of the fastest moving ejecta. Since day 1787, the spectral index has remained constant at α = -0.95 ± 0.04 (S ∝ να). These observations are all consistent with the shock having encountered a denser shocked component of the progenitor's stellar wind. At the current rate of expansion, the shock is expected to encounter the inner optical ring in the year 2006 ± 3, in line with predictions made by hydrodynamic simulations. Using superresolution, we have also obtained 9 GHz images of the remnant (resolution ≈ 05) at four epochs. The emission is distributed around the rim of a nearly circular shell, but has become increasingly asymmetric with time. There are two "hot spots" to the east and west, aligned along the major axis of the optical ring. This morphology is most likely indicative of an axisymmetric circumstellar medium into which the shock is expanding, consistent with present understanding of the progenitor star and its environment. The two hot spots are increasing in flux density at different rates, which may indicate directional anisotropies in the ejecta. We believe that the northern and southern regions of the remnant are encountering a shocked wind that is less dense and also further from the progenitor star than that in the bright regions of emission. As a result, these regions should eventually brighten and/or extend.
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