EROS2 : Light Echoes from SN1987a

After a journey of ~160000 years, a bright flash have been observed in 1987. It came directly from the location of the supernovae, called SN1987a. But light that has been emitted in others direction could have encounter on its way some nearby interstellar clouds and then been reflected. In that case, some luminous arcs could be observed around the supernovae, they are called 'light echoes'. The two main arcs or rings observed are due to clouds located at a distance of about 300 and 1100 light-years in front of the supernovae.

The EROS2 experiment has monitored during almost 7 years the Large Magellanic Cloud, and has then been able to observe the evolution in time of these echoes. As time goes on, these echoes appear to expand outward from the center, as we see on the animated image (fig. 1) and on figure 3. Due to the geometry, an apparent superluminal proper motion is visible : ring's diameter seems to grow up with a velocity higher than many light-year per year. In fact, these rings are not matter, it's a geometrical effect, a play of light that make us feel this. The evolution of the arcs are due to many different light rays that have followed distinct paths from the supernovae, with a small angular shift between each others. This could be understand with the figure 2 and the 2 blue rays : the upper one have a longer path to do before it reach us, and will then appear after the lower one.