Supernova outbreak: X rays signal earliest alert

Science News,  March 8, 2008  by Ron Cowen

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Thanks to a lucky break and an overactive galaxy, astronomers have for the first time caught a massive star in the act of exploding. An X-ray outburst recently recorded by NASA's Swift satellite suggests that researchers began viewing the violent demise of a star in the galaxy NGC 2770 just a few seconds after the first X rays arrived at Earth, and hours before the first visible-light fireworks.

Most supernovas aren't identified until they generate an outpouring of visible light, long after key information about the collapsing star has vanished. The new finding suggests that astronomers using wide-angle X-ray telescopes could routinely witness the very beginnings of hundreds of supernova explosions each year, suggest Alicia Soderberg and colleagues in an online posting.

Such early, X-ray signs of supernovas have been predicted for 4 decades but never before been found.

On Jan. 9, Soderberg and her colleagues were using an X-ray telescope on Swift to study a supernova in NGC 2770 that had been discovered 10 days earlier. Just as Swift began observations of this supernova, it recorded a fresh spike of X rays from another region in the galaxy that lasted for about 7 minutes. On Jan. 11, using the Gemini North telescope on Hawaii's Mauna Kea, Soderberg and her colleagues identified the visible-light fingerprint of the new supernova, now dubbed SN 2008d, in NGC 2770.

Soderberg, of Princeton University and the California Institute of Technology in Pasadena, and her team posted the findings at http://xxx.lanl.gov/abs/0802.1712 on Feb. 13. They declined comment because they have submitted the article to Nature.

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The team suggests that the outburst reflects the nature of the star that exploded. When stars more than eight times the sun's mass succumb to gravity, their interiors collapse, giving birth either to a neutron star or to a black hole. Tens of seconds after the collapse, a shock wave reaches the still unperturbed surface of the star and the region just beyond. It's in this relatively low-density environment that the energy locked inside the shock can finally be released as high-energy radiation, or X rays.

From the energy and duration of the initial X-ray release, the researchers suggest that the exploded star was compact but surrounded by a substantial stellar wind, hurled out before it went supernova.

Observing a supernova so early in the game shows "what the progenitor star [was like] just before the explosion" comments Roger Chevalier of the University of Virginia in Charlottesville.

Even these early X rays arrive seconds after other emissions--specifically, neutrinos and gravitational waves that emerge from the supernova's core, notes Andrew MacFadyen of New York University. However, an early X-ray alert would allow researchers to rapidly, if retroactively, determine when and where to look for these crucial, core emissions. This ability offers the strongest promise for revealing the inner workings of supernova explosions.

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