Intermediate · 2:14

Magnetar

The strongest magnets known, and the day one flashed across the galaxy

Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.

Below are the 10 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the Phenomena tab

1. A magnet the size of a city

A magnetar is a neutron star with the strongest magnetic field known. SGR 1806-20's is about 2×10¹⁵ gauss: four million billion times Earth's.

A neutron star is what is left when the core of a massive star collapses: about 1.4 times the Sun's mass in a ball some 24 km across. Most have magnetic fields around a trillion times Earth's. About 30 of the roughly 3,000 neutron stars known have fields a hundred to a thousand times stronger still. They are called magnetars.

SGR 1806-20, about 28,000 light-years away in Sagittarius, is one of the strongest. Its field is about 2×10¹⁵ gauss. Earth's is half a gauss, a fridge magnet's about a hundred, and the strongest steady magnet ever built, at the US National High Magnetic Field Laboratory, 455,000.

So close to a magnetar, atoms themselves are squeezed into thin needles along the field. Even 1,000 km away the field would still be a few billion gauss, enough to distort the atoms of anything alive.

2. A slow, braking spin

It turns once every 7.55 seconds, slowly for a neutron star, and its field brakes it: each turn takes 0.016 seconds longer every year.

Most young neutron stars spin many times a second. Magnetars turn slowly, once every 2 to 12 seconds, because their enormous fields act as brakes: as the star turns, its field whirls charged particles out into space, and they carry its spin away.

SGR 1806-20 turns once every 7.55 seconds, and each turn takes about 0.016 seconds longer than a year before. How fast a star slows tells how strong its field must be, and that is how its field is measured: about 2×10¹⁵ gauss. Robert Duncan and Christopher Thompson predicted such stars in 1992, and in 1998 the slowing of SGR 1806-20 showed they were real.

3. A twisted field

Inside, the field is thought to be stronger still and tangled. As it works its way out, it twists the field lines above the star and strains the solid crust they are anchored in.

A neutron star's crust is a solid shell about a kilometre thick, a lattice of iron-rich nuclei far stronger than steel. The magnetic field runs through it and is anchored in it.

In magnetars the field is thought to be even stronger inside the star, and tangled. Over years it slowly pushes its way out, dragging the crust and twisting the field lines that loop above the surface. Twisted field lines carry electric currents, and the currents heat the crust: magnetars glow in X-rays more brightly than their slowing spin could pay for. It is their magnetism, not their spin, that powers them.

The twisting shown here is drawn to show the idea, as the leading explanation has it (Thompson, Lyutikov and Kulkarni 2002). No one can see the field itself.

4. Starquakes

When the strain is too much, the crust cracks. Each starquake shakes the field and sends out a burst of X-rays and gamma rays lasting about a tenth of a second.

Magnetars were first noticed for these bursts. In 1979 satellites caught short, repeated bursts of gamma rays from the same patches of sky, and their sources were named soft gamma repeaters: the SGR in SGR 1806-20.

A typical burst lasts about a tenth of a second and gives off as much energy as the Sun does in days to months, mostly as X-rays. When a magnetar wakes up it can fire hundreds of them over weeks, then go quiet for years. The favoured explanation is that the crust breaks under the field's strain, like the ground in an earthquake, and the field lines above it snap into a new shape, letting their energy go.

The crack and the bursts are drawn, slowed down six times.

5. The giant flare

On 27 December 2004, SGR 1806-20 let go at once. For a fifth of a second it outshone the full Moon at Earth, 28,000 light-years away: the brightest flash ever seen from beyond the Solar System.

The giant flare reached Earth at 21:30 UT. It was almost all gamma rays, invisible to the eye, but for a fifth of a second it brought more energy to Earth than the full Moon's light does (Palmer et al. 2005). It overwhelmed nearly every gamma-ray detector in space and even disturbed Earth's upper atmosphere, the ionosphere.

In that fifth of a second the magnetar gave out about as much energy as the Sun does in 56,000 years. Only two giant flares had been seen before, from other magnetars in 1979 and 1998, and this one was about a hundred times stronger.

It is thought to have come when a large part of the field, strained for years, broke and snapped into a new shape at once, throwing off a cloud of matter at a good fraction of the speed of light.

The flash is gamma rays, drawn in pale violet, slowed down about five times.

6. A pulsing tail

After the flash, a fireball held down by the field glowed for about six minutes, brightening each time the star's turn, every 7.56 seconds, carried it towards us.

The first flash lasted a fifth of a second. What followed was a softer glow that faded over about 380 seconds, rising and falling every 7.56 seconds: the star's own turn.

The explanation is a trapped fireball: a ball of electrons, positrons and gamma rays at hundreds of millions of degrees, caught in the closed loops of the field just above the surface. Its own pressure would blow it apart in a split second; only a field this strong can hold it. As it cooled from the outside in, its glow faded, while the star's turning swung it towards Earth and away again.

Here the fireball is drawn on one side of the star; its true shape is not known. The tail runs about four times faster than it did.

7. The star rang

In the tail, the X-rays flickered at steady pitches, from 18 to 1,840 times a second: the crust was still shaking from the quake, like a struck bell.

Satellites that timed the tail's X-rays closely found them flickering at fixed frequencies: 18, 26, 30, 92, 150, 625 and 1,840 times a second (Israel et al. 2005; Strohmayer and Watts 2006). A similar set had been found in the 1998 giant flare of SGR 1900+14.

They match the ways a neutron star's crust can twist back and forth, one half against the other, just as seismologists read the ringing of the Earth after an earthquake to learn what is inside it. It was the first time the inside of a neutron star could be sounded out like this.

These frequencies are pitches we can hear. With sound on, they play here as tones; their loudness is drawn. The crust's twisting is drawn slowed down so the eye can follow it.

8. Heavy elements

The flare threw off matter too. About ten minutes later a faint glow of gamma rays showed that it held freshly made heavy elements: about a third of Earth's mass.

The flare threw off a cloud of matter rich in neutrons at a good fraction of the speed of light. Looking again at the old records, astronomers found a later, fainter glow of gamma rays that peaked about ten minutes after the flash.

Its brightness and timing match the radioactive decay of freshly made heavy nuclei: the rapid capture of neutrons, the r-process, that makes gold, platinum and uranium. About a millionth of the Sun's mass was made, a third of Earth's (Patel et al. 2025). Giant flares like this could make one to ten percent of the heaviest elements in the Milky Way; most of the rest comes from neutron stars colliding.

The cloud is drawn as a shell glowing gold; its true shape is not known.

9. A radio burst from our galaxy

On 28 April 2020 another magnetar, SGR 1935+2154, gave out an X-ray burst and a flash of radio waves shorter than a thousandth of a second: the first fast radio burst seen from inside the Milky Way.

Fast radio bursts are flashes of radio waves lasting a few thousandths of a second, first found in 2007. Thousands have been seen, almost all from other galaxies, and what makes them was a mystery.

On 28 April 2020 two radio telescopes, CHIME in Canada and STARE2 in the United States, caught one from SGR 1935+2154, about 30,000 light-years away, at the same moment as an X-ray burst from it. It came as two flashes 29 thousandths of a second apart. Seen from another galaxy it would have looked like a faint fast radio burst: at least some of those faraway flashes come from magnetars.

Radio waves are invisible; here they are drawn in green, slowed down about thirty times.

10. Fading away

Magnetars are young: their fields wear down within about 10,000 years. Then the bursts stop, and the star lives on as a quiet neutron star.

About 30 magnetars are known, all in the Milky Way or its companion the Large Magellanic Cloud. They lie near young star clusters and the wreckage of supernovae, and few are more than a few thousand years old. One, in the cluster Westerlund 1 16,000 light-years away, was born from a star about 40 times the Sun's mass.

As its field decays and pushes out its twists, it heats the star and powers the bursts. After some ten thousand years there is too little left to crack the crust. The magnetar slows, cools and fades into one of the hundreds of millions of quiet neutron stars thought to fill our galaxy.

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