Tuesday, September 05, 2006

Flashing out a Star's Demise

Astronomers may for the first time have caught a star in the act of blowing itself to smithereens. The cataclysmic supernova announced itself by producing an unusually long flash of x-rays. The new find should help researchers better understand the violent deaths of the most massive stars in the universe.
When a rapidly rotating massive star explodes at the end of its life, it leaves a black hole as a corpse. Such explosions blow out jets of matter in two opposite directions and produce brief bursts of energetic gamma rays. These classical gamma ray bursts (GRBs) are the most powerful cosmic explosions since the big bang. Astronomers have long suspected that milder explosions, known as x-ray flashes, are related to GRBs. But it wasn't clear whether x-ray flashes really represent less energetic GRBs or are just normal GRBs seen from a different angle.

The new observation has settled the mystery. On 18 February, NASA's Swift satellite detected a 35-minute x-ray flash now known as XRF 060218(see ultraviolet image above). Immediately, the satellite pointed its x-ray and ultraviolet telescopes toward the explosion, which occurred at an unusually close distance to Earth-a mere 440 million light years away in the constellation Aries the Ram. Ground-based observatories were alerted within minutes, and just a few days later, big telescopes on the ground and in space had a look. "These are by far the best supernova observations ever made," says Swift principal investigator Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The analysis of the observations, penned in four papers in this week's issue of Nature, "proves that x-ray flashes and gamma ray bursts share the same origin," says astrophysicist Elena Pian of the University of Trieste, Italy, who co-authored two of the papers. Moreover, the power of the accompanying supernova explosion (catalogued as SN 2006aj) was somewhere between typical supernovas and extremely energetic hypernovas that are associated with GRBs. This suggests there is a whole continuum of related phenomena, says Gehrels, although he admits there's no clue yet why some bursts are less energetic than others.

One team of authors, led by Alicia Soderberg of the California Institute of Technology in Pasadena, suggests that normal supernovas leave a compact neutron star as the explosion remnant, while GRBs leave a black hole. X-ray flashes are somehow in between: They leave a highly magnetized neutron star (known as a magnetar), whose energy produces GRB-like jets of matter moving close to the speed of light, only less energetic.

Another team, lead by Sergio Campana of the Brera Observatory in Merate, Italy, claims it has evidence that Swift actually saw the onset of the star's break-up. But supernova theoretician Adam Burrows of the University of Arizona, Tucson, is doubtful. "It's not clear what has been seen," he says, adding that "a lot more data and much better theories" are needed to finally solve the explosion mechanisms of very massive stars.


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