A MYSTERIOUS X-ray glow that surrounds the sun may be evidence for the existence of an exotic particle that physicists have been hunting for decades.
Astronomers have been puzzled by the sun’s X-ray halo since it was first detected in the 1940s. Curiosity deepened when the Japanese satellite Yohkoh, launched in 1991, sent back X-ray pictures showing spectacular flares streaming from sunspots and a gentle glow emanating from the sun’s outer atmosphere.
But the surface of the sun is not hot enough to produce such a bright X-ray glow. So where are the X-rays coming from? Konstantin Zioutas and his colleagues think that heavyweight particles called axions could be the source.
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Zioutas, a theorist who works at the University of Thessaloniki in Greece and the CERN particle physics laboratory in Geneva, Switzerland, suggests that the X-rays are produced by the decay of axions. According to his team’s model, axions are created in the hot core of the sun and expelled, only to become trapped by the sun’s gravity. The physicists have calculated the rate at which axions might accumulate around the sun and combined it with an estimate of how quickly they might decay. This predicts how the brightness of the X-ray halo should change with increasing distance from the centre of the sun.
In a paper to be published in The Astrophysical Journal next month, Zioutas and his colleagues report that the predictions match brightness measurements made by the Yohkoh satellite.
The catch is that no one is sure axions even exist. Axions were dreamed up in the 1970s to explain differences between the way nuclear forces behave in experiments, and the way theories predict they should. The search for them intensified in the 1980s when cosmologists realised that axions could be the missing dark matter that holds the universe together. But they are predicted to interact with other matter only weakly and no axions have ever been detected.
Have Zioutas and his colleagues finally managed to pin them down? “It’s exciting,” says Pierre Sikivie, a theorist in the physics department at the University of Florida in Gainesville, “but I don’t think the evidence presented can, at this point, be considered proof that axions exist.” There may be a simpler explanation for the origin of the solar X-rays.
Until all the alternatives have been ruled out, says Leslie Rosenberg, head of an axion-hunting experiment at Lawrence Livermore National Laboratory in California, assuming that axions are responsible for the sun’s X-ray glow is “like coming home, seeing the door to your house open and saying, ‘Oh my God, Martians must have been here'”. It’s not wrong, but it is wildly speculative.
Rosenberg also cautions that Zioutas’s model relies on a type of axion that can only exist in a universe with more than four dimensions – and so far we have no evidence for extra dimensions in ours.