Speckle interferometric observations of supernova 1987A and of a bright associated source

Abstract

The explosion of SN1987A in the Large Magellanic Cloud¹ provides a unique opportunity for the direct measurement, by means of interferometry, of the spatial distribution and evolution of the optical emission of a supernova. In particular, such observations may help to resolve the controversy concerning the physics of the hydrogen emission-line region^2,3. Also, as the supernova fades there is the prospect of obtaining the intensity structure of any light echo arising from dust-scattered radiation⁴. Here we report optical speckle interferometric observations of SN1987A on days 38, 47 and 50 after the explosion on 23 February 1987. The supernova itself was unresolved, with angular radius upper limits of ∼0.012 arc s at 3,869 Å, 0.015 arc s at 4,861 Å and 4,921 Å, and 0.020 arc s at 5,875 Å and 6,585 Å, consistent with angular radii derived from photometry and spectroscopy. On day 30 after the explosion a group from the Harvard–Smithsonian Center for Astrophysics (Cf A) discovered a second source lying at a small angular displacement from the supernova^5,6. They again observed it on day 38. On day 50 we observed a second source at 6,585 Å. We measure its offset from the supernova to have been 0.074 ± 0.008arcs and estimate that it was ∼3mag fainter than the supernova at 6,585 Å at the same epoch. Such a bright source was not present before the explosion, implying that it was caused by the supernova. The source we observed was similar in magnitude and position angle to the feature discovered by the Cf A group, although we find the angular displacement to have been 0.015 ± 0.011 arcs larger.