In recent years, light echoes have been discovered around some nearby extragalactic supernovae well after the explosion, most notably the light echoes from Supernova 87A (SN 87A). However, to date no light echoes of historical Supernovae (SNe) of Galactic or extragalactic origin have been discovered. The SuperMACHO collaboration has discovered light echoes in the Large Magellanic Cloud (LMC) which are associated with ancient supernova remnants (SNR) listed as three of the six youngest SNRs in the LMC. These three remnants are also identified as Type Ia events, based on the X-ray spectral abundances. We have dated these echoes at 400-600 years.
The echoes were detected in difference images of digital data from the SuperMACHO Survey. These difference images are images from one epoch subtracted from another epoch. The difference images removed all the static artifacts (stars and nebulae) and only variable objects such as variable stars and echoes remain in the data. Below is an example that shows the power of difference imaging with one of the light echo rings from SN 87A. In the difference image, white represents flux enhancements in the original image and black in the template.
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Click on the image below to see an animation of all the rings from SN 87A: template image and the 4 difference images spanning 4 years.
Besides the well known echoes from SN 87A we found three further light echo groups that are not associated with SN 87A. For each of these light echoes, we estimated a vector direction as shown in the Figure below.
If these vectors are extrapolated backward in time, they point to three well-defined positions as the origins of the echo complexes.
The echoes are generally (but not always) highly superluminal, and have surface brightnesses ranging from 22 to our limit of ~24 mag/arcsec2 in our custom VR filter. The difference images removed all the static artifacts (stars and nebulae) and only variable objects such as variable stars and echoes remain in the data. We have developed a very sophisticated pipeline to take, reduce, and search the MOSAIC data for variable objects. By combining the position and apparent proper motions, we can derive rough ages for the echoes. For 1987A we derive an age of 15.9 ± 1.4 yr, consistent with the real age of 14.8 yr. For two of the remaining 3 echoes, we derive ages of 610 and 410 yrs. The positions of the three new echo centers correspond to 3 of the 6 youngest SNRs in the LMC, and from X-ray data, these SNRs probably came from Type Ia explosions.
12/08/2005
01/07/2002