Supernova 1987A: radiosphere resolved with VLBI five days after the neutrino burst

Following the detection¹ of radio emission from SN1987A in the Large Magellanic Cloud, we conducted very-long-baseline inter-ferometry (VLBI) observations with an interferometer composed of a NASA Deep Space Network antenna (DSS42) at Tidbinbilla, Australia and the antenna of the Hartebeesthoek Radio Astronomy Observatory, South Africa^2,3. We did not detect any emission from the supernova above a level of ̃20% of the supernova's total flux density, although signals were detected from our two calibrator sources with amplitudes roughly equal to those determined in earlier VLBI observations. We infer that we resolved the supernova's radiosphere and estimate, for an epoch 5.2 days after the neutrino burst^4,5, a lower bound on the radiosphere's radius of 1.2 mas. Given the photometric data from the supernova^6,7, a distance to the Large Magellanic Cloud of 50 ± 5 kpc (ref. 8), and an apparent expansion velocity that varied systematically with time from 18-16 x 10³ km s^-1 (refs 9 and 10), as estimated from the blue-shifted Hα absorption lines on the days preceding our observations, we conclude that 5.2 days after the neutrino burst the supernova's radiosphere was at least 2.5 times larger than the inferred blackbody photosphere, and at least as large as the Hα line-forming region.