Magnetar

All neutron stars are strongly magnetic, but magnetars are in a class of their own. Their fields reach 10⁹ to 10¹¹ tesla, trillions of times Earth’s and a hundred million times stronger than any magnet ever built. The decay of that field powers bursts of X-rays and gamma rays.

Dangerous from far away

A field this strong does strange things. From about 1 000 kilometres away it would distort the electron clouds of atoms in your body, killing you. From halfway to the Moon, it could wipe the magnetic stripe of every credit card on Earth.

The flare of 2004

On 27 December 2004, a magnetar called SGR 1806−20, tens of thousands of light-years away across the Milky Way, released a giant flare. In a tenth of a second it emitted around 10⁴⁰ joules, more energy than the Sun gives off in many thousands of years. The gamma rays were strong enough to reach Earth and briefly expand our upper atmosphere, the ionosphere. Giant flares like this are thought to come from starquakes, when the star’s crust cracks under the strain of its twisting magnetic field. Only a few have ever been recorded, in 1979, 1998 and 2004.

Short lives

Robert Duncan and Christopher Thompson proposed magnetars in 1992. They estimated that about one supernova in ten leaves one behind. The extreme field does not last: after roughly 10 000 years it decays, and the magnetar fades into an ordinary neutron star. About 30 active magnetars are known, but tens of millions of burnt-out ones may lie hidden in our galaxy.

Fast radio bursts

Fast radio bursts are millisecond flashes of radio waves from far across the universe, and their source has been a mystery. On 28 April 2020 a magnetar in our own galaxy, SGR 1935+2154, produced a pair of bright radio bursts, raising the possibility that magnetars are behind at least some of them.