Magnetars and gamma-ray bursts

What is the strongest magnet there is? A dead star. When a heavy star blows up, its centre can be crushed into a neutron star: a ball about 20 km across that weighs more than the Sun. Every neutron star is strongly magnetic. About one in ten is thought to be born far stronger still, and is called a magnetar. A strong magnet you can hold has a strength of about 1 on the scale that scientists use. A magnetar can reach 100 billion.

Nothing on the Earth comes close, and nothing could survive near one. From 1,000 km away a magnetar’s field would pull the atoms of your body out of shape, so that the chemistry of life stopped working. From half way to the Moon it could wipe every bank card on the Earth. Only about thirty are known, all thousands of light years away. One of them sits inside a ring of warm dust seven light years across.

A field that strong puts the star’s hard crust under enormous strain. Now and then the crust cracks. This is a starquake, and it lets out a flash of gamma rays, the most energetic kind of light. On 27 December 2004 a magnetar on the far side of our galaxy flared. In a tenth of a second it gave out as much energy as the Sun does in 150,000 years. It was more than 40,000 light years away, and the flash still jolted the top of the Earth’s air.

There are bigger flashes yet. In 1967 American satellites built to watch for secret nuclear bomb tests picked up bursts of gamma rays coming from space. Satellites now catch about one a day. Each comes from a far galaxy and, for a few seconds, can outshine everything else in the sky in gamma rays. Most are the death of a very heavy star whose centre falls in to make a black hole. Two thin jets punch out through the star at nearly the speed of light, and a burst is seen only if one jet points at us.

The brightest ever recorded reached the Earth on 9 October 2022. Astronomers call it the BOAT, for brightest of all time. It came from 2.4 billion light years away and still disturbed the top of the Earth’s air as much as a flare on the Sun. Its X-rays lit up sheets of dust in our own galaxy as rings. A burst that bright and that close may come only once in ten thousand years.

Some bursts last under two seconds and have a different cause: two neutron stars spiralling together and colliding. In August 2017 one of these was caught twice, first as a ripple in space itself and 1.7 seconds later as a flash of gamma rays. The glow that followed showed newly made heavy elements, gold among them. No burst has ever been seen to go off in our own galaxy.