Back in 2005, Armin Rest from Harvard and a few mates, spotted the echo of a supernova in the Large Magellanic Cloud. The explosion had kicked off some 900 years ago but what Rest and co were seeing was its reflection from cold dark dust in the cloud. Since then, the team has even measured the spectrum of the echo to determine that it was Type 1a supernova
Impressive work but Team Rest have not been idle. They have been busy hunting for other echos and today it looks as if they’ve come up trumps. Instead of looking for echos from the Large Magellanic Cloud, the team looked at the Milky Way (a harder task because it takes up a much larger portion of the sky) and found numerous clusters of echos from two recent supernova: Tycho (SN1572) observed by the Danish astronomer Tycho Brahe in 1572 and Cassiopeia A, which went more or less unobserved when it exploded in 1667.
This is important for astronomers because it gives them access to a a kind of exotic wayback machine. All of a sudden, they can study the physics of these supernovae at the moment they exploded and compare that with the properties of the remnants today.What’s more, by looking for echoes of different supernovae that have reflected off the same dustcloud, they have a way to directly measure the distance between them. They conclude that Cassiopeia A is (probably) 1900 light years further away from us than Tycho.
So it provides a new way to measure distance too.
The team has now begun a program to look for the echoes from five other supernova that have occured in the last 2000 years or so. Expect to hear more about that in the near future
Ref: arxiv.org/abs/0805.4607: Scattered-light Echoes from the Historical Galactic Supernovae Cassiopeia A and Tycho (SN 1572)