Communiqué de presse

Light Echoes from Supernova 1987A in the LMC: Snapshots of Interstellar Clouds

16 mars 1988

New and exciting observations at the European Southern Observatory have probed the interstellar space around last year's bright supernova SN 1987A in the Large Magellanic Cloud (LMC)

On images obtained with the ESO 3.6 m telescope in mid-February 1988, light echoes [1] have been unambiguously detected, which arise from the reflection of the bright supernova light in interstellar clouds surrounding the exploded star.

Light echoes from SN 1987A

When, on February 23, 1987, observations were first made of the exploding supernova SN 1987A in the LMC (see eso8704, eso8705, eso8706 and eso8707), the initial, very bright light-flash of the first few seconds was already past. But some of this light, travelling away from the supernova in other directions, may be reflected from dust clouds in the vicinity of the supernova. A small part of that reflected light will be directed towards the Earth and can be seen after some time as a faint “light echo" of the original flash.

Astronomers have been searching for this phenomenon since last year. A detection on March 3, 1988, was announced by A. Crotts from McDonald Observatory, Texas, U.S.A.. Earlier images were made at the European Southern Observatory on January 25 by H. Pedersen with the Danish 1.5 m telescope and on February 13, by M. Rosa with the ESO 3.6 m telescope. In particular the latter observations unambiguously show a double light echo (see attached photo).

For these exposures of SN 1987A with the EFOSC instrument at the ESO 3.6 m telescope, the bright light from the supernova itself was dimmed by the insertion of a small, obscuring disk into the instrument. Two almost concentric rings are clearly visible with radii of 32 and 51 arcseconds. The rings are brighter towards North (top of photo), probably because there is more interstellar matter in this direction. The intensity of the light echo is more than 10,000 times fainter than the current brightness of the supernova and is in agreement with the predicted intensity.

From the time delay of about one year and the angular dimensions of these rings, it can be inferred that the reflecting clouds are at distances of approximately 400 and 1000 light years in front of the supernova, respectively.

Continued observations of these light echoes will allow to determine the three-dimensional structure of the interstellar clouds near the line-of-sight to the supernova. The rings are expected to expand by about 5% of their present diameter per month and variations in the brightness along the periphery will indicate variations in the density of the reflecting material.

For this reason, direct and spectral observations of the light echoes have been initiated at ESO. Pictures in different colours were obtained last night with the 3.6 m telescope by Chr. Gouiffes (ESO) and M. T. Ruiz (Universidad de Chile). A quick check of some photographic plates, obtained with the ESO Schmidt telescope towards the end of 1987, appears to indicate that it will become possible to follow the development of the echoes further back in time.

The reductions of these observations are rather time consuming and further results will be reported as soon as they become available.

Notes

[1] This Press Release is accompanied by a B/W photo, showing a double light echo.

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ESO
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