Miércoles 29 de Noviembre de 2006, Ip nº 181

Dark-matter particles could 'X-ray' the Sun
Por Marcus Chown

An intriguing particle first glimpsed last year, which could be related to particles that make up the universe's dark matter, might help us see right through the sun.

The particle in question is the axion. It was originally proposed to fix a problem with the strong force in particle physics, but has more recently been considered as a candidate for dark matter, the mysterious, unseen stuff thought to make up nearly 90 per cent of galaxies' mass. Last year an experiment at the Legnaro National Laboratory in Legnaro, Italy, provided tantalising hints of an axion, but it interacted too strongly with matter to be a good fit for dark matter (New Scientist, 15 July, p 35).

However, the axions found by the Italian team would still interact so rarely with normal matter that they would be able to slip right through even the super-dense plasma of the sun. Now Malcolm Fairbairn of Stockholm University in Sweden and his colleagues say this could allow such axions to "X-ray" the sun.

The team base their claim on a model of how gamma-ray photons interact with the sun's magnetic field. This showed that when gamma rays pass through the sun's outer layers, a small fraction would be transformed into axions. These particles would easily penetrate the sun and emerge on the far side, where the magnetic field would flip a small fraction of the axions back to gamma-ray photons. "If this happens, it means that even if a gamma-ray source is eclipsed by the sun, we should still be able to see it, just as if the sun is partially transparent," Fairbairn says (www.arxiv.org/astro-ph/0610844).

In 1991, the quasar 3C 279 was eclipsed by the sun while astronomers were observing it using the orbiting Energetic Gamma Ray Experiment Telescope (EGRET). While gamma rays were seen during the occultation, the detectors on the instrument were nowhere near discriminating enough to pin them uniquely to 3C 279.

Fairbairn thinks that the much more accurate Gamma-Ray Large Area Space Telescope (GLAST), to be launched in 2007, will be able to keep tabs on a gamma-ray source as it goes behind the sun. "Observing [such] gamma rays would confirm the existence of an entirely new particle," says Fairbairn. "Potentially it might allow us to 'X-ray' the interior of the sun," he adds.

Nigel Smith of the Rutherford Appleton Laboratory in the UK and a member of the UK Dark Matter Collaboration is intrigued. "The calculations - especially on the magnetic fields that the axion would be exposed to - look rather complex, but the idea certainly looks sound," he says.

  27/11/2006. New Scientist Magazine.