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Gamma rays from a nova explosion greet astronomers with surprise

Fermi's Large Area 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). The images show the rate of gamma rays with energies greater than 100 million electron volts (100 MeV); brighter colors indicate higher rates.
Credit: NASA/DOE/Fermi LAT Collaboration

Recent observations from a telescope in orbit around Earth have detected gamma rays coming from a nova explosion on a distant star system, known as V407 Cygni. Experts say that this is a surprising discovery because scientists have expected to observe X-rays, instead of gamma rays, coming from similar nova explosions.

Novas like this one are actually thermonuclear explosions on the surfaces of white dwarf stars that are locked into binary systems with other stars. They are different from supernova explosions because they do not completely destroy the white dwarf stars.

Japanese amateur astronomers discovered Nova Cygni 2010 in an image taken at 19:08 UT on March 10 (4:08 a.m. Japan Standard Time, March 11). 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.
Credit: K. Nishiyama and F. Kabashima/H. Maehara, Kyoto Univ.

Aous Abdo and colleagues at the Fermi-LAT Collaboration used the Fermi Large Area Telescope to observe these gamma rays being emitted from the V407 Cygni binary star system—a binary system that consists of a white dwarf and a red giant star.

The discovery was unexpected, but the researchers suggest that the expanding "shell" of the nova explosion must have interacted with the space around the nearby red giant star in order to produce the gamma rays.

Very few binary systems involving white dwarf stars are known to have similar environments and produce such gamma rays, so these researchers propose that gamma-ray novae are quite rare in the universe.