The re-brightening of supernova 1987A

Josefin Larsson is one of the Oskar Klein Center postdocs since august 2009, working at the Astronomy department. Her recent paper, X-ray illumination of the ejecta of supernova 1987A, written in collaboration with several other people from the OKC, has been accepted in Nature and has been published online last week.
I asked Josefin to write something for this blog about the exciting results she found. Here is her account.

Supernova 1987A exploded in our neighbouring galaxy the Large Magellanic Cloud nearly 25 years ago. Due to its proximity it can be studied in much greater detail than any other supernova. This has lead to many important discoveries over the years, and even now, many years later, it continues to surprise us.

SN1987A - Josefin Larsson

The Hubble Space Telescope has observed SN 1987A regularly since the mid-nineties (a selection of images is shown above) and we have recently used these data to measure how the brightness of the ejecta changes over time. Our analysis revealed that the flux declined up to around 2001, or about 5000 days after the explosion, but then started to increase, more than doubling by the end of 2009. By modelling the light curve we found that the declining phase is well explained by energy input from the radioactive decay of 44Ti. However, radioactivity cannot explain the re-brightening, so a new energy source must have started to dominate around 2001.

The brightening turns out to be connected to the interaction between the ejecta and the ring of circumstellar gas. As the outer parts of the ejecta slam into the ring, it brightens across the entire electromagnetic spectrum. The X-ray emission produced in this process then illuminates the inner part of the ejecta, causing the observed brightening. This change in the dominant energy source, from radioactive decay to interaction with the circumstellar medium, can be seen as the transition from supernova to supernova remnant.

A very nice consequence of the X-ray illumination is of course that the supernova will be bright enough to study for many years to come. In time the X-rays will also penetrate different layers of the ejecta, enabling us to analyse the structure and chemistry of the exploded star. This will be an exciting form of X-ray tomography.

Our findings were published online in Nature this week and will appear in print on 23 June. Here you can read the paper online: X-ray illumination of the ejecta of supernova 1987A

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