Back in 2001, astronomer Franz Bauer noticed a bright, variable source in the Circinus spiral galaxy, using NASA’s Chandra X-ray Observatory. Although the source displayed some exceptional properties Bauer and his Penn State colleagues could not identify its nature confidently at the time. But now, seven years later Bauer and his team have confirmed this object was a supernova. By combining online data in the public archives from 18 different ground- and space-based telescopes, one of the nearest supernovae in the last 25 years, SN1996cr, has finally been identified. “It’s a bit of a coup to find SN 1996cr like this, and we could never have nailed it without the serendipitous data taken by all of these telescopes. We’ve truly entered a new era of ‘internet astronomy’,” said Bauer.
Because this object was found in an interesting nearby galaxy, the public archives of these telescopes contained abundant observations. Clues from a spectrum obtained by ESO’s Very Large Telescope led Bauer and his team to start the real detective work of searching through data from various telescopes.
The data show that SN 1996cr is among the brightest supernovae ever seen in radio and X-rays. Visible-light images from the archives of the Anglo-Australian Telescope in Australia show that SN 1996cr exploded sometime between 28 February 1995 and 15 March 1996, but it is the only one of the five nearest supernovae of the last 25 years that was not seen shortly after the explosion.
It also bears many striking similarities to the famous supernova SN 1987A, which occurred in a neighbouring galaxy only 160,000 light-years from Earth. Until now, it was the only known supernova with and X-ray output that increased over time. SN1996cr has the same attributes, but is much brighter.
“This supernova appears to be a wild cousin of SN 1987A,” says Bauer. “The two look alike in many ways, except this newer supernova is intrinsically a thousand times brighter in radio and X-rays.”
The combined data, in conjunction with theoretical work, have led the team to develop a model for the explosion. Before the parent star exploded, it cleared out a large cavity in the surrounding gas, either via a strong wind or from an outburst from the star late in its life. So the blast wave from the explosion itself could expand relatively unimpeded into this cavity. Once the blast wave hit the dense material surrounding SN1996cr, the impact caused the system to glow brightly in X-ray and radio emission. The X-ray and radio emission from SN 1987A is probably fainter because the surrounding material is less compact.
Astronomers think that both SN 1987A and SN 1996cr show evidence for these pre-explosion clear-outs by a star doomed to explode. Having two nearby examples suggests that this type of activity could be relatively common during the death of massive stars.
“Not only does our work suggest that SN 1987A isn’t as unusual as previously thought, but it also teaches us more about the tremendous upheavals that massive stars can undergo over their lifetimes,” said co-author Vikram Dwarkadas of the University of Chicago.
So all you internet astronomers, get out there and start clicking! Who knows what you’ll find.