Echoing supernova.

Echoing supernova

Infrared observations of supernova 1987A show a bright ring around the supernova that is apparently an infrared echo of it. The recently completed analysis of those observations, which were made Aug. 6 at the European Southern Observatory in La Silla, Chile, were reported in "International Astronomical Union Circular 4481' by A. Chalabaev of the Observatoire de Haute Provence at St.-Michel-l'Observatoire, France, and C. Perrier and J.M. Mariotti of the Observatoire de Lyon (France).

At the same time, in the Dec. 1 ASTROPHYSICAL JOURNAL LETTERS, Bradley E. Schaefer of the NASA/Goddard Space Flight Center in Greenbelt, Md., predicts the appearance of an echo in visible light. This visible echo should in fact already be there, he says, although it is indistinguishable because of the brightness of the supernova itself. Later, as the supernova dims, the echo should become more dominant. By a year after the supernova explosion--around March 1988--the echo should be a ring of about 10th magnitude, visible in small telescopes and maybe binoculars. It should persist for decades. "After the supernova itself has faded, the light-echo nebula will remain visible like the Cheshire Cat's smile,' Schaefer writes.

On earth, the first indication of a supernova is light coming directly from the exploding star. However, the exploding star is surrounded by diffuse interstellar matter and may also have a somewhat denser circumstellar shell of material that sometime in the past came off the star itself. Light going out in all directions from such an explosion will be reflected off this matter, and some of the reflected light will eventually come to earth. This reflection, the "echo,' will first appear sometime after the supernova itself, as the light takes times to go out and get reflected. The geometry of the situation is such that at any particular time, a ring of matter at a particular distance and angle from the supernova will reflect the light seen on earth as the echo, accounting for the ring shape.

The echo is interesting not only as an optical phenomenon but also as a probe of the nature of the matter surrounding the supernova. Terrestrial telescopes cannot resolve the light echoes of distant supernovas, says Schaefer, but two fairly close novas have exhibited the effect, Nova Persei 1901 and Nova Sagittarii 1936.