Solar flare largest ever seen

The most colossal x-ray flare ever detected has been caught in the act of zapping its solar system with planet-killing radiation.

The star is II Pegasi in the constellation Pegasus, about 135 light-years from Earth.

That means the explosive flare seen by the NASA Swift satellite, designed to detect much more distant and powerful gamma-ray bursts, took place around the year 1871. Light from the event is only now reaching Earth.

The x-ray flare is the first-ever detected beyond our own Sun that bears a striking resemblance to the much smaller 'x-class' flares generated occasionally by our own Sun.

"It's a hundred thousand times more powerful than the largest solar flares ever recorded," says astronomer Dr Steven Drake of NASA's Goddard Space Flight Center.

Despite being far more powerful, it looks like it was created in the same way, he says.

It starts with a tangle of magnetic field lines on the surface of a star that short-circuits. When that happens, atomic particles are accelerated to speeds only seen on Earth in high-tech particle accelerators.

The accelerated particles can emit gamma rays, which is what caught the Swift satellite's attention in the first place.

When the satellite turned to face II Pegasi, it took aim with its x-ray detector and caught the hour-long eruption of x-rays.

Violent eruption

The x-rays were created as material violently erupted from the star and then arched back down and slammed back onto its surface.

By comparison, x-ray flares on the Sun last only second or minutes, at most.

"It's certainly one of the biggest ever seen," says Drake about the II Pegasi flare. It's the hands-down winner in terms of those seen in 'soft' x-rays, which are the rays just beyond the wavelength of extreme ultraviolet light.

Drake is a co-author on a paper on the flare that is being presented by the University of Maryland's Dr Rachel Osten at a meeting in Pasadena this week.

Flaming star

What's less surprising about the flare, however, is that it originated from II Pegasi, says Pennsylvania State University astrophysicist Professor Eric Feigelson.

"It's known to be one of the most flaring stars," says Feigelson. If he had been asked to guess which nearby stars were capable of belting out such a flare, II Pegasi would have been among his top 10, he says.

Despite being a middle-aged star that ought to be past this sort of wild and violent behaviour, II Pegasi is part of a tightly-bound two-star system in which the stars are roaring around each other, generating powerful tidal forces that keep II Pegasi riled up.

Fortunately our own Sun is relatively quiet and stable, with x-ray flares that are unable to penetrate Earth's atmosphere.