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Superbright supernova
first of its kind

Posted: December 4, 2009

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An extraordinarily bright and long-lasting supernova represents one of the first examples of the population of stars that first sprung into life in the early Universe.

Known as SN2007bi, this supernova occurred in a nearby dwarf galaxy and was discovered by the international Nearby Supernova Factory (SNfactory) based at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. The supernova's unusual spectrum caught the astronomers' attention and over the last year and a half they have attained even more data as the supernova slowly faded away.

In this schematic illustration of the material ejected from SN 2007bi, the radioactive nickel core (white) decays to cobalt, emitting gamma rays and positrons that excite surrounding layers (textured yellow) rich in heavy elements like iron. The outer layers (dark shadow) are lighter elements such as oxygen and carbon, where any helium must reside, which remain unilluminated and do not contribute to the visible spectrum.

The analysis indicated that the supernova’s progenitor star could only have been a massive star weighing at least 200 times the mass of our Sun and initially containing few elements besides hydrogen and helium, very much like the first stellar inhabitants of the Universe.

“Because the core alone was some 100 solar masses, the long-hypothesized phenomenon called pair instability must have occurred,” says astrophysicist Peter Nugent. “In the extreme heat of the star’s interior, energetic gamma rays created pairs of electrons and positrons, which bled off the pressure that sustained the core against collapse.”

Instead of turning into a black hole like many other heavyweight stars, its core went through a nuclear runaway that blew it to shreds. This type of behavior has been predicted, but this is the first convincing observation of the process in action. The SNfactory has already discovered nearly a thousand Type Ia supernovae – the 'standard candles' used to study the expansion history of the Universe – but SN 2007bi was at least ten times as bright, and is thought to represent the first confirmed observation of a pair-instability supernova.

“The thermonuclear runaway experienced by the core of SN 2007bi is reminiscent of that seen in the explosions of white dwarfs as Type Ia supernovae,” says Alex Filippenko, “but on a much larger scale and with a far greater amount of power.”

Follow up observations were made with the Keck telescope and the Very Large Telescope (VLT) in Chile as SN 2007bi slowly faded over the course of 555 days. “The Keck and VLT spectra clearly indicated that an extremely large amount of material was ejected by the explosion, including a record amount of radioactive nickel, which caused the expanding gases to glow very brightly,” says Paolo Mazzali from the Max Planck Institute for Astrophysics in Garching, Germany.

Computer simulations were composed to try and replicate the properties observed in this supernova explosion. Rollin Thomas of Berkeley's Computational Research Division aided the early analysis, using the Franklin supercomputer at the National Energy Research Scientific Computing Center (NERSC). “The code uses hundreds of cores to systematically test a large number of simplified model supernovae, searching through the candidates by adjusting parameters until it finds a good fit,” he says. “This kind of data-driven approach is key to helping us understand new types of transients for which no reliable theoretical predictions yet exist.”

The model fit was unambiguous: SN 2007bi was a pair-instability supernova.

“The central part of the huge star had fused to oxygen near the end of its life, and was very hot,” Filippenko explains. “Then the most energetic photons of light turned into electron-positron pairs, robbing the core of pressure and causing it to collapse. This led to a nuclear runaway explosion that created a large amount of radioactive nickel, whose decay energized the ejected gas and kept the supernova visible for a long time.”

The team say that it is significant to have found the first unambiguous example of a pair-instability supernova in a dwarf galaxy because they are so faint and contain few elements heavier than hydrogen and helium, but this makes them ideal "fossil laboratories" to study the early Universe.

“In the future, we might end up detecting the very first generation of stars, early in the history of the Universe, through explosions such as that of SN 2007bi – long before we have the capability of directly seeing the pre-explosion stars,” adds Filippenko.

The researchers discuss their results in the 3 December 2009 issue of Nature.

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