Gamma radiation from Nova outbreaks

High-energy radiation bursts in novae apparently rather rule than exception

Artistic representation of a classic nova in a binary system. © NASA's Goddard Space Flight Center / S. Singer pointed
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Exception or rule? A Nova caused a surprise among astronomers with a gamma-ray outbreak several years ago. Now it turns out: The gamma-ray Nova is not so extraordinary. Three more star explosions of this kind sent the high-energy radiation also into space, report astronomers in the journal "Science". Thus, this variety of radiation could be a normal part of the classic novae.

From the Latin "Stella nova" - new star - derives the name "Nova" from: A star that suddenly reappears in the sky. However, these are not really new stars: A classic nova arises when a white dwarf star in a binary star system subtracts enough matter from its companion star to ignite a new thermonuclear explosion on its surface. The brightness of the star increases significantly over a period of a few days or weeks, so that sometimes even previously invisible stars suddenly shine bright. Such astronomical events are not so rare: each year they occur about 30 to 50 times in our Milky Way.

Surprising high-energy Nova

Since the novae are so common, they are quite well studied - so far thought. However, about four years ago, measurements with the Fermi Space Telescope caused a surprise: the binary system V407 Cygnus emitted high-energy gamma radiation in addition to normal light during a nova outbreak. It was thus initially another enigmatic gamma ray source. "Until then, no one had considered the possibility that even novae could accelerate particles into the gigaelectronvolt range and thus be visible in the light of gamma radiation, " says Olaf Reimer of the University of Innsbruck.

That such gamma outbreaks are no exception at Novae, confirmed now more data from the Fermi telescope. Three more gamma-ray Nova occurred in 2012 and 2013. This confirmation is especially important because the Nova of V407 Cygnus itself seems to be an exceptional case: the two partner stars orbit here much more closely than in classic Novae. In addition, the material donor is a red giant, from which a lot of material flows to his partner star. This leads to much more frequent outbreaks in the range of decades instead of centuries.

The four previously observed gamma rays Novae, the colors show the strength of gamma radiation (blue low, yellow high). NASA / DOE / Fermi LAT Collaboration

The three newer observed Novae V1324 Sco, V959 Mon, and V339 Del, however, all belong to the otherwise comparatively unspectacular classical type. They took place in the constellations Scorpio, Unicorn and Dolphin. Remarkably, they resemble each other in many ways: The observed spectra and energies of gamma radiation are very similar. For all radiation bursts, the gamma-ray emission lasted two to three weeks, but did not start until several days after the novae were already visible in visible light. display

Gamma rays in all classic novae?

"We can now hypothesize that all classic novae are associated with high-energy gamma-ray emission, " says Reimer. The great difference between the many observations in visible light and the four novae in the gamma-ray field over a period of about five years is explained by the sensitivity of the telescopes at different wavelengths. Thus, the novae in the gamma-ray range are observed only from the comparatively small distance of nine to fifteen thousand years light years to our earth. However, the typical rates of several dozen novae per year occur over the entire 100, 000 to 120, 000 light-years across the Milky Way.

The scientists are not yet able to say clearly which physical processes produce the gigaelectronvolt gamma radiation in Novae. "Unfortunately, we do not know the conditions well enough or are not yet observing them precisely enough to make that decision, " says Reimer. However, with the recent observations of the Fermi gamma-ray telescope, the exclusive club of extreme high-energy sources in space has received several new members.

(Science, 2014; doi: 10.1126 / science.1253947)

(University of Innsbruck, 01.08.2014 - AKR)