Sunday, March 9, 2008

Supernova Outbreak: X rays signal earliest alert

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.

BANG! Astronomers serendipitously found the supernova SN 2008d, located in the galaxy NGC 2770, through X-ray observations (labeled as XRF080109). This observation marks the first finding of signs of a stellar explosion emitted so soon after a star's demise. The two other supernovas labeled here were found in this galaxy last year and in 1999.
CREDIT: A. de Ugarte Postigo/ESO et al, Dark Cosmology Centre/Univ. of Copenhagen, Instituto de Astrofísica de Andalucía (CSIC), and Univ. of Hertfordshire

Most supernovas aren't identified until they generate an outpouring of visible light, long after key information about the size and other properties of 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 her colleagues in an online posting.

Astronomers have previously observed the immediate aftermath of a related class of stellar explosions, the demise of massive stars that are signaled by gamma-ray bursts, the most powerful eruptions in the universe. But in each of the four cases for which researchers have firmly established the link between a supernova and a gamma-ray burst, the bright visible-light afterglows of the bursts hide the information-rich early emissions from the supernovas, Soderberg's team notes.

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 collaborators posted their findings at on Feb. 13. They declined to comment for this story because they have submitted the article to Nature.

The team suggests that the outburst reflects the nature of the star that exploded. When stars more than about eight times the sun's mass succumb to gravity, their interiors implode, 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 intensity and duration of the initial X-ray release, the researchers suggest that the star that exploded was compact but is surrounded by a substantial stellar wind, hurled by the star before it went supernova. The researchers suggest that the supernova belongs to a class known as Ibc, characterized by low abundances of hydrogen and silicon. Unlike the rare type of Ibc supernovas that produce gamma-ray bursts, this supernova is rich in helium.

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.

"We haven't observed any supernova as early as SN2008d," says Yizhong Fan of the University of Copenhagen's Niels Bohr Institute and the Purple Mountain Observatory in Nanjing, China. "SN 2008d provides us a unique chance to see what happens when an Ibc supernova is born and then to constrain the underlying physics." Fan and his colleagues recently posted their own interpretation of the X-ray data online ( They propose that the X-ray outburst is a lower-energy analog of a gamma-ray burst.

Li-Xin Lin of the Max Planck Institute for Astrophysics in Garching, Germany, comes to a similar conclusion in an article he posted online March 4 ( The energy and duration of the X-ray outburst indicates the supernova "is an extension of the gamma-ray burst-supernova connection toward the low energy end," he says. "It also strengthens the GRB-supernova connection."

Even the early X rays recorded by Swift would arrive 10 seconds or more after other emissions—specifically, neutrinos and gravitational waves that emerge from the supernova's core, notes Andrew MacFadyen of New York University in New York City. However, an early X-ray alert would allow researchers to rapidly, if retroactively, determine exactly where and when 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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