Super-blast amazes scientists

Astronomers using NASA’s Fermi Gamma-ray Space Telescope have detected gamma-rays from a nova for the first time, a finding that stunned observers and theorists alike.

The discovery overturns the notion that novae explosions lack the power to emit such high-energy radiation.

A nova is a sudden, short-lived brightening of an otherwise inconspicuous white dwarf star. The outburst occurs when a white dwarf in a binary system erupts in an enormous thermonuclear explosion.

“In human terms, this was an immensely powerful eruption, equivalent to about 1,000 times the energy emitted by the Sun every year,” said Elizabeth Hays, a Fermi deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“But compared to other cosmic events Fermi sees, it was quite modest. We’re amazed that Fermi detected it so strongly.”

Gamma rays are the most energetic form of light, and Fermi’s Large Area Telescope detected the nova for 15 days. Scientists believe the emission arose as a 1.5 million-kilometre-per-hour shock wave raced from the site of the explosion.

before and after images showing nova

Japanese amateur astronomers discovered Nova Cygni 2010 in an image taken on March 11 (Japan Standard Time). The erupting star (circled) was 10 times brighter than it was in an image taken several days earlier.

Discovered by Japanese amateurs

Japanese amateur astronomers first noticed that the star system, known as V407 Cyg, was 10 times brighter on March 11, 2010, than in an image they had taken three days earlier.

It was quickly followed up by other Japanese amateurs, and then by professional astronomers working with Fermi’s Large Area Telescope (LAT).

V407 Cyg lies 9,000 light-years away. The system is a so-called symbiotic binary containing a compact white dwarf and a red giant star about 500 times the size of the Sun.

“The red giant is so swollen that its outermost atmosphere is just leaking away into space,” said Adam Hill at Joseph Fourier University in Grenoble, France. The phenomenon is similar to the solar wind produced by the sun, but the flow is much stronger.

“Each decade, the red giant sheds enough hydrogen gas to equal the mass of Earth,” Hill added.

The white dwarf intercepts and captures some of this gas, which accumulates on its surface. As the gas piles on over tens to hundreds of years, it eventually becomes hot and dense enough to fuse into helium. This energy-producing process triggers a runaway reaction that explodes the accumulated gas.

The white dwarf itself, however, remains intact.

Fermi gamma ray image showing V407 Cyg nova

The Large Area Telescope aboard NASA's Fermi Gamma-ray Space Telescope saw no sign of a nova in 19 days of data prior to March 10 (left), but the eruption is obvious in data from the following 19 days (right).

A huge shock, in more ways than one

The blast created a hot, dense expanding shell called a shock front, composed of high-speed particles, ionised gas and magnetic fields. The shock wave expanded at 11 million kilometres per hour—or nearly 1 percent the speed of light.

The magnetic fields trapped particles within the shell and whipped them up to tremendous energies. Before they could escape, the particles had reached velocities near the speed of light. Scientists say that the gamma rays likely resulted when these accelerated particles smashed into the red giant’s 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,” said NRL’s Soebur Razzaque.

Supernovae remnants can last for 100,000 years and affect regions of space thousands of light-years across.

Adapted from information issued by Francis Reddy, NASA’s Goddard Space Flight Centre. Images courtesy NASA / DOE / Fermi LAT Collaboration / K. Nishiyama and F. Kabashima / H. Maehara, Kyoto Univ. / GSFC / Conceptual Image Lab.

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