Origin of the pearl necklace of SN1987A
  The bright circles observed around stars are usually considered as produced by shock waves; but this interpretation does not explain easily the bright spots of the "pearl necklace" of NS 1987A supernova.   Assuming that the central object of SN 1987A is a neutron star heated by the accretion of a low density cloud, non-linear optics, in particular superradiance and impulsive stimulated Raman scattering (ISRS), is needed to take into account the high intensity of the radiated light.   Where the temperature of the surrounding gas decreases enough to allow a combination of protons and electrons into atomic hydrogen in despite of the low density, a spherical shell absorbs in particular the Lyman alpha line, but does not populate much the 2P state because a tangential superradiance appears until the exciting line is almost absorbed; the increase of the 2P population resulting from the disappearance of the superradiance produces a redshift, so that almost all energy of a wide band is transferred to tangential modes making an UV pearl necklace in a given direction of observation.   In a column of UV light making a pearl, atomic lines are excited enough to produce new, co-linear superradiances, in particular visible. 
Jacques Moret-Bailly