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Surprise gamma-ray
blast from nova

Posted: 13 August 2010

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The idea that nova explosions lack the power to emit high-energy radiation has been dramatically overturned with the surprise detection of gamma-rays blasting from a nova in the constellation Cygnus.

The story began on 10 March with Japanese observers Koichi Nishiyama and Fujio Kabashima, who noted a ten-fold increase in brightness of the V407 Cyg system compared with images they had taken three days earlier. They alerted the wider community and the observation was independently confirmed. Both NASA's Fermi Gamma-ray Space Telescope and Swift were also pointed at the site.

Japanese amateur astronomers discovered Nova Cygni 2010 in an image taken on March 10. The erupting star (circled) was 10 times brighter than in an image taken several days earlier. The nova reached a peak brightness of magnitude 6.9, just below the threshold of naked-eye visibility. Image: K. Nishiyama and F. Kabashima/H. Maehara, Kyoto Univ.

Fermi monitored the so-called nova for 15 days. “In human terms, this was an immensely powerful eruption, equivalent to about 1,000 times the energy emitted by the Sun every year,” says Fermi scientist Elizabeth Hays. “But compared to other cosmic events Fermi sees, it was quite modest. We’re amazed that Fermi detected it so strongly.”

Novae are dramatic stellar outbursts caused by the accretion of hydrogen onto a white dwarf star sparking nuclear fusion, but are not usually powerful enough to generate high-energy radiation. In the case of the V407 Cyg system, which is in a binary configuration comprising a compact white dwarf and a red giant star about 500 times the size of the Sun, scientists think that the emission arose as a seven million-mile-per-hour shock wave raced from the site of the explosion.

The nova eruption is clear in the Fermi data, which shows the rate of gamma rays with energies greater than 100 million electron volts – brighter colours represent higher rates. Image: NASA/DOE/Fermi LAT Collaboration.

“The red giant is so swollen that its outermost atmosphere is just leaking away into space,” says Adam Hill at Joseph Fourier University in Grenoble, France. As the gas piles on to the companion white dwarf over decades and centuries, it eventually becomes dense enough to fuse into helium. An explosion results, propelling the accumulated gas as a shell of churning high-speed particles, ionized gas and twisted magnetic fields out from the white dwarf. The observed gamma-rays likely occurred as a result of the accelerated particles smashing into the red giant's stellar wind.

“We know that the remnants of much more powerful supernova explosions can trap and accelerate particles like this, but no one suspected that the magnetic fields in novae were strong enough to do it as well,” says Soebur Razzaque of the Naval Research Laboratory.

“It takes thousands of years for supernova remnants to evolve, but with this nova we’ve watched the same kinds of changes over just a few days,” adds colleague Kent Wood.

A paper detailing the discovery features in the 13 August edition of the journal Science.