Revenge of the Magnetar

Solar flares that scorch Earth's atmosphere are commonplace. But scientists have discovered a few each year that are not like the others: they come from stars thousands of light years away.

A solar flare blasts hot gas away from the limb of the Sun.

On August 24th, 1998, there was an explosion on the sun as powerful as a hundred million hydrogen bombs. Earth-orbiting satellites registered a surge of x-rays. Minutes later they were pelted by fast-moving solar protons. Our planet's magnetic field recoiled from the onslaught, and ham radio operators experienced a strong shortwave blackout.

None of these things made headlines. The explosion was an "X-class" solar flare, and during years around solar maximum, such as 1998, such flares are commonplace. They happen every few days or weeks. The August 24th event was powerful, yet typical.

A few days later - no surprise - another blast wave swept past Earth. Satellites registered a surge of x-rays and gamma-rays. Hams experienced another blackout. It seemed like another X-class solar flare. Except for one thing: this flare didn't come from the sun.

It came from outer space.

"The source of the blast was SGR 1900+14 - a neutron star about 45,000 light years away," says NASA astronomer Pete Woods. "It was the strongest burst of cosmic x-rays and gamma rays we've ever recorded."

SGR 1900+14 is a special kind of neutron star called a magnetar. "Magnetars have the strongest magnetic fields in the universe: a million billion (10¹⁵) gauss," he says. For comparison, the magnetic field of the sun is less than 10 gauss in most places, and about 1000 gauss near sunspots.

An artist's concept of a magnetar outburst. The red loops trace the star's intense magnetic field.

Magnetism and solar flares go together. On the sun, flares happen when magnetic fields above sunspots become twisted and stretched. They're like rubber bands pulled too tightly. Snap! They recoil with explosive results. Physicists call this "magnetic reconnection."

Physicist Maxim Lyutikov of McGill University thinks the same thing happens on magnetars. "I imagine that the atmosphere of a magnetar is similar to the solar corona - filled with plasma and complicated magnetic fields," he says. "Reconnection on the sun is often caused by a plasma instability called the 'tearing mode.' Detailed calculations show that a similar instability may develop in the strongly magnetized plasma of a magnetar."