Figure 1 : Animated Color gif images of the light echoes.

• 'Raw Mpeg' (~7.1 Mo) and 'Mov' (~6.4 Mo) version (complete and high quality)

• 'Avi' (~4.2 Mo) and 'Mp4' (~0.7 Mo) version (complete and lower quality)

• Gif version (~24 Mo, complete and high quality)

• Gif version (~7.8 Mo, 1/3 images and high quality)

The echoes appear to expand outward from the center (SN1987a). About 1200 real EROS2 images from july 1996 to february 2002 (~6.7 yrs) have been used for this animation in the complete version. The sampling is lower during the first 3 years due to the observational strategy.

The movie has been compiled with real images (it is not a simulation) : we used our 2 bands (called Red (~I filters) and Blue (V and R filter)) and a third colour has been created (linear association of the 2 others) to make a "true" colour movie. Many details, other than the light echoes, can be seen : a variable star in the top left corner (a Mira), clouds in the LMC, a Bok globul that is visible during a short time (north of SN87a) due to a higher surface brightness. The red colour that appears in the bottom right corner is due to a CCD artefact in the Red band.

The Supernovae SN1987a is the most famoust supernovae known. It appeared the 23rd of February 1987, in the Large Magellanic Cloud, a dwarf galaxy satellite of our Milky Way, close to the Tarentula nebulae. It has been studied many times during the past 20 years. See these links for more details : Wikipedia(en) , Wikipedia(fr) .

One impressive phenomena related to it, are the light echoes. The principle is simply explained by the drawing below.

Figure 2 :

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.

Figure 3 : Zoom on a raw differential image obtained by subtracting 2 images separeted by ~5 years (9th february 1997 and 16th march 2002). Two kind of arcs appear. The dark ones are echoes observed in 2002, while the bright ones have been seen at the beginning of the experience.
Look carefully! We can also guess the presence of a nearby star (~30 pc) close to the center of the field, due to its proper motion (~390 milli-arcsec per year)... : there is a bright point followed by a dark one! This motion can't be seen on the movie because of a blending effect, there is a LMC bright star at the same position.