False colour image of the supernova SN2002bj (blue) on top of its host galaxy, NGC1821.
Credit: D. Poznanski; W. Li; and A.V. Filippenko
SYDNEY: Astronomers have discovered a new type of supernova - the thermonuclear blast from a dying star - which happens three to four times faster than other known types.
Writing in the U.S. journal Science, the researchers, led by astronomer Dovi Poznanski from the University of California, Berkeley, said it is the fastest evolving supernova they have ever seen.
"It was three to four times faster than a standard supernova, basically disappearing within 20 days. Its brightness just dropped like a rock," he said.
Bright flash
Poznanski dug up images of the supernova, dubbed SN 2002bj, in data gathered seven years ago by the Katzman Automatic Imaging Telescope (KAIT) at the Lick Observatory near San Jose, California.
The supernova's bright flash appeared in several images of the galaxy NGC 1821, in the constellation Lepus, but was originally classified as a Type II supernova and filed away.
But its rapid evolution, dimness, strong signature of helium and absence of hydrogen have led astronomers to reclass the supernova as a new variety. They have called it a 'point Ia supernova', because it is one-tenth as bright and one tenth as long lasting as a more common Type Ia supernova.
Images of the sky before, during and after the supernova showed that it brightened and dimmed in just 27 days, whereas most supernovae shine for three to four months.
New explosion mechanism
Astronomer and co-author Alex Filippenko, also from UC Berkeley, says the supernova "may well be a new physical explosion mechanism, not just a minor variation of ones already known."
"This supernova is qualitatively different from the complete disruption of a white dwarf, known as a Type Ia supernova, or the collapse of an iron core and rebound of the surrounding material, so-called 'core-collapse supernovae.'"
The explosion may have been triggered in a binary star system as helium from a white dwarf star was accreted onto another white dwarf companion, triggering an explosion sufficient to power a faint but rapid explosion.
Fortuitously, this type of explosion was modelled in 2007 by astrophysicists led by Lars Bildsten at the Kavli Institute for Theoretical Physics at UC Santa Barbara. Their theoretical model of a rapid supernova was published in the Astrophysical Journal Letters.
Unlike a regular Type Ia explosion, both white dwarfs survive the detonation of the primary white dwarf's helium shell in this rapid supernova.
Fillipenki says the explosion has similarities to both a supernova and a nova, a brightening of a star due to a sudden explosion. However the explosion would have generated about 1000 times the energy of a standard nova, said Fillipenko, and was more like a supernova.
"A thicker, more massive layer of helium undergoes nuclear runaway than in the case of an ordinary nova, so it is a lot more powerful comparable to that of a somewhat low-power supernova," Fillipenko told Cosmos Online.
Hint of rare metal
Unusual in the spectral signature of the supernova is the possible presence of the element vanadium - a metal which is found in a small percentage of meteorites but has never been detected in a supernova before.
"We need to study the detailed properties of these supernovae by observing many of them; after all, any given object might be atypical or peculiar. But they are clearly rare objects," he said.
Astronomer Brian Schmidt, from the Australian National University, in Canberra, said it was the first time a new type of explosion had been realised in decades and that new, wide-sky surveys, such as the Australian SkyMapper telescope, which became operation in May this year, should find more of these faint objects.
"The name fits because it's an explosion like the Ia supernova, it just doesn't have the same oomph," he said.
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