A light echo around supernova 2003gd
Light echo from SN 1987A in the Large Magellanic Cloud
CREDIT: Anglo-Australian Observatory, photograph by David Malin
Yesterday there was a paper describing the discovery of a light echo from supernova 2003gd in the nearby galaxy M74. Light echoes are often bizarre looking effects such as the image on the left which shows the light echo from supernova 1987A, an explosion observed by astronomers in the Large Magellanic Cloud (LMC) in 1987. Of course the explosion really happened about 170,000 years ago but the light took that long to reach us due to the distance between the LMC and us.
A light echo forms when the light from the explosion, traveling out in all directions, is reflected by clouds of material (dust and gas) in the interstellar medium (ISM), the "stuff" between the stars. First we see the light from the explosion itself, but later we see the reflected light from these clouds of material around the star which has taken longer to get here because it has travelled further. By measuring the time delay between seeing the explosion and seeing the light echo we can estimate how far from the clouds are from the site of the explosion. The Hubble website has a nice little animation which shows how this effect works. Another famous light echo which you may have seen is that from V838 Mon, a nova which occured in 2002.
So why is 2003gd interesting, and why am I writing about it so early on a Saturday morning?! Well, the answer to the second bit is just that I can't sleep, I've been up for two hours. More importantly, supernova light echoes are rare. This is only the fifth one to ever be detected. They are difficult to find for several reasons. Firstly they are faint, the reflection is much dimmer than the initial explosion, so you need a good telescope and a long observation in order to find them, especially in such distant objects. Most supernova we have seen in recent years (with the exception of 1987A) have occured in galaxies which are several megaparsecs away. You also need a large telescope in order to see enough detail to resolve the light echo. The futher away a supernova is, the smaller the angular size of the echo, so the more resolution you will need in order to see it.
Light echoes allow us to investigate the ISM around the supernova in detail. By watching them carefully we can determine how tenuous the ISM is, what it's made of (by using spectroscopy), and how clumpy it is by the structure we see in the echo. All of this helps the theorists who spend their time modeling stellar evolution. In the case of 2003gd, the observations show that the ISM has a similar density to that in our own Galaxy (one or two particles per cubic centimetre), but is made up of smaller dust grains (less than a quarter of a micron on average). Hopefully, more observations will be carried out using the Hubble Space Telescope over the next few years so that more of the structure can be observed as the light echo moves out from the site of the explosion.