Magnetars

What is a magnetar?

When a massive star explodes as a supernova, it can leave behind a neutron star: a city-sized ball with more mass than the Sun crammed into about 20 km. A magnetar is a neutron star that, for a few thousand years, has an absolutely monstrous magnetic field - around a thousand times stronger than an ordinary neutron star, and roughly a quadrillion (10¹⁵) times stronger than Earth's.

There are only around 30 confirmed magnetars in our galaxy so far, though astronomers think there could be many more that have gone quiet. They were first proposed by scientists Robert Duncan and Christopher Thompson in 1992, and confirmed by observations in the years after.

Just how strong is the magnetic field?

Magnetic field strength is measured in a unit called gauss. The numbers are so spread out that I had to use a special scale (each step is 10× bigger) to even fit them on screen. Click a button to compare:

How magnetars form

This part is still being researched, but the leading idea goes like this:

1. A very massive star runs out of fuel and its core collapses in a supernova.
2. The collapsing core is born spinning extremely fast - maybe hundreds of times a second.
3. If the new neutron star spins fast enough and churns inside (a dynamo effect, a bit like what makes Earth's magnetic field, but turbocharged), it can build up a magnetic field a thousand times stronger than normal.
4. That gives you a magnetar. The intense field then slowly "brakes" the spin, which is why magnetars actually rotate quite slowly - usually once every 2 to 12 seconds.

A magnetar, drawn with code

Soft gamma repeaters & starquakes

A magnetar's magnetic field is so strong it actually strains the solid crust of the neutron star. Every so often the crust cracks - a starquake - and the snapping magnetic field blasts out a burst of gamma rays and X-rays. Magnetars that do this are called soft gamma repeaters (SGRs), because they repeatedly send out "soft" (lower-energy) gamma rays.

Another group, historically called anomalous X-ray pulsars (AXPs), glow steadily in X-rays. Astronomers now think SGRs and AXPs are basically the same thing - both are magnetars - just caught behaving in different ways.

Giant flares - the universe showing off

Once in a while, a magnetar releases something far bigger than a normal burst: a giant flare. These are some of the most powerful explosions in the galaxy.

This is where magnetars overlap with gamma-ray bursts, the brightest explosions known. Most long gamma-ray bursts come from supernovae and merging stars, but magnetar giant flares can look like short gamma-ray bursts from far away - so astronomers have to be careful telling them apart.

Magnetars vs pulsars vs ordinary neutron stars

This confused me at first, so here's the way I finally understood it. All three are neutron stars - they're just different "personalities" of the same kind of object:

So: every magnetar and every pulsar is a neutron star. Magnetars are just the rare, super-magnetic members of the family.

Current scientific research

Magnetars are a really active research area right now. Some of the big questions scientists are working on:

• Do fast radio bursts mostly come from magnetars? The 2020 event from SGR 1935+2154 suggested some do, but maybe not all.
• Exactly how does a magnetar build such a strong field - is it the dynamo idea, or something about the original star?
• What happens deep inside, where matter is denser than an atomic nucleus? Magnetars are natural physics labs we could never build on Earth.
• Telescopes and satellites like NASA's NICER, Fermi and Swift, and ESA's X-ray missions, watch magnetars for bursts.