What Is a Magnetar? The Most Extreme Object in the Universe

A magnetar is a supernova. About 20 km wide but containing more mass than the Sun.">neutron star — the collapsed core of a massive star — with a magnetic field so strong that the field itself, not the star's rotation, powers most of what we see from it. Earth's magnetic field at the surface is roughly 25–65 microtesla. A hospital MRI scanner runs at 1.5–3 tesla. The strongest steady magnetic fields made in laboratories are around 45 tesla. A magnetar's surface field is typically 10^10 to 10^11 tesla — up to a hundred billion tesla.

Fields that strong do strange things. Theorist Robert Duncan, who co-proposed the magnetar model in 1992, pointed out that fields above about 10^5 tesla would be instantly lethal to a human, because they distort the electron clouds in the molecules of living tissue. By that measure, a magnetar would kill you through pure magnetism from about 1,000 km away, before its X-rays and gamma rays even entered the calculation.

This article is about what makes magnetars different from other neutron stars: the origin of their fields, the starquakes and giant flares they produce, their role in solving the fast-radio-burst puzzle, and how astronomers find and catalogue them.

Key takeaways

• Magnetars are neutron stars with surface fields of about 10^10–10^11 tesla, roughly 100–1,000 times stronger than typical radio pulsars.
• Their energy comes from the decay of the magnetic field, not from spinning down — that is the defining difference from ordinary pulsars.
• They spin slowly, with periods of about 1 to 12 seconds, and are young: active for perhaps 10,000 years.
• The McGill Online Magnetar Catalog lists 30 magnetars (24 confirmed, 6 candidates) as of its November 2020 update.
• On 27 December 2004, SGR 1806-20 released about 2 × 10^39 joules in a giant flare, the brightest extrasolar event ever recorded.
• On 28 April 2020, SGR 1935+2154 emitted a fast radio burst, the first ever traced to a known object in our galaxy.

Neutron stars in one paragraph

When a star of roughly 8 to 20–25 solar masses runs out of fuel, its iron core collapses in under a second and the outer layers are blown off in a core-collapse supernova (see our guide to supernova types). The surviving core is a ball of mostly neutrons about 20–25 km across, with 1.4 to about 2 solar masses packed inside it, reaching densities around 10^17 kg per cubic metre. Most neutron stars are seen as pulsars, whose radio beams sweep past Earth like a lighthouse — first discovered by Jocelyn Bell Burnell in 1967, as told in the Bell Burnell pulsar discovery. For the full picture of neutron-star structure, see Neutron Stars: The Most Extreme Objects in the Universe. Here we focus on the magnetic subset.

What makes a magnetar different?

Ordinary radio pulsars have surface fields of around 10^8 tesla (10^12 gauss). Magnetars sit in a different regime, with fields of 10^10–10^11 tesla (10^14–10^15 gauss). In the McGill catalogue's estimates, SGR 1806-20 tops the list at about 2 × 10^15 gauss, or 2 × 10^11 tesla.

But the field strength is not the formal definition. What really separates a magnetar is its energy budget. A normal pulsar shines by tapping its rotation: as it spins down, rotational energy is converted into radiation. A magnetar's X-ray output is often far greater than its spin-down could supply. The extra energy has to come from somewhere, and the only reservoir big enough is the magnetic field itself, slowly decaying and twisting.

Property	Typical radio pulsar	Millisecond pulsar	Magnetar
Surface magnetic field	~10^8 T	~10^4–10^5 T	~10^10–10^11 T
Spin period	~0.1–few seconds	1–10 milliseconds	~1–12 seconds
Main energy source	Rotation	Rotation (spun up by accretion)	Magnetic field decay
Typical age	Up to tens of millions of years	Up to billions of years	~10^3–10^4 years (active phase)
Signature	Steady radio pulses	Extremely stable radio pulses	X-ray bursts, outbursts, giant flares

The spin periods are worth noting. The McGill catalogue's magnetars range from about 1.36 seconds (Swift J1818.0-1607) to about 11.8 seconds (1E 1841-045). Their huge fields act as a brake, transferring angular momentum to the surrounding plasma and slowing them quickly. That slow, rapidly lengthening spin period is itself a fingerprint: combine the period with its rate of change, and you get an estimate of the field strength.

Where do magnetar fields come from?

Nobody has watched a magnetar being born, so the origin of the field is still debated. Three ideas dominate.

The dynamo model. Robert Duncan and Christopher Thompson proposed in 1992 that if a newborn neutron star spins very fast — with a period of a few milliseconds — during its first seconds, violent convection inside it could act as a dynamo, amplifying the field enormously. Earth's liquid outer core generates our planet's field in a broadly similar way, though on a scale many orders of magnitude weaker.

The fossil-field model. A second idea is that the progenitor star was already strongly magnetic. When its core collapses from thousands of kilometres to about 10 km, magnetic flux is conserved and the field strength rises dramatically, roughly with the inverse square of the radius.

Magnetorotational instabilities. Computer simulations of core collapse show that turbulence driven by differential rotation can also amplify the field, a process sometimes combined with the dynamo picture.

These models make testable predictions. If dynamo action requires very fast initial spin, then magnetar births should release extra rotational energy into their supernovae, and some astronomers have linked magnetars to unusually energetic "superluminous" supernovae and to some long gamma-ray bursts. Those links are plausible but not settled.

Physics at 10^11 tesla

At these field strengths, effects that are negligible on Earth become dominant.

Distorted atoms. Electrons in a normal atom occupy orbitals shaped mainly by electric attraction to the nucleus. In fields above about 10^5 tesla, the magnetic force on electrons begins to compete with that attraction. At magnetar strengths, it dominates: atoms are squeezed into thin needle-like shapes aligned along the field lines. Chemistry on a magnetar's surface would bear little resemblance to chemistry on Earth.

The quantum critical field. Physicists define a "quantum critical" field, about 4.4 × 10^9 tesla, at which the energy spacing of an electron's quantised orbits in the field equals its rest-mass energy. Many magnetars exceed this value. Above it, quantum electrodynamics (QED) predicts effects that are otherwise unobservable.

Vacuum birefringence. QED predicts that a strong field makes empty space itself behave like a crystal, so light polarised in different directions travels slightly differently. In 2016 astronomers using ESO's Very Large Telescope reported polarisation from the isolated neutron star RX J1856.5-3754 consistent with this effect. In 2022, NASA's Imaging X-ray Polarimetry Explorer (IXPE) measured strongly polarised X-rays from the magnetar 4U 0142+61, which are consistent with radiation passing through an extremely magnetised environment. Neither result is a laboratory-grade measurement, but both point the same way.

Photon splitting. In fields near and above the critical value, a single high-energy photon can split into two lower-energy photons. This QED process is thought to shape the hard X-ray spectra of magnetars by limiting how energetic their escaping photons can be.

How a magnetar would affect you at different distances

A magnetar's field falls off roughly as a dipole, with strength proportional to 1/r³. Using a surface field of 10^11 tesla at a radius of about 10 km gives the following rough figures:

Distance from the magnetar	Approximate field	What it means
Surface (10 km)	~10^11 T	Atoms deformed into needles
1,000 km	~10^5 T	Lethal to humans through magnetic forces on tissue (Duncan's estimate)
10,000 km	~100 T	Twice the strongest lab magnets; destroys electronics and pulls ferrous metal violently
384,000 km (Earth–Moon distance)	~2 × 10^-3 T	About 40 times Earth's field; roughly fridge-magnet strength
1 astronomical unit (150 million km)	~3 × 10^-11 T	Negligible magnetically

These are our own order-of-magnitude dipole calculations, intended to show scale. The magnetic field is not what makes a magnetar dangerous at astronomical distances; its radiation is. A giant flare from a magnetar a few light-years away would deliver an enormous dose of gamma rays and could deplete Earth's ozone layer. No known magnetar is close enough to pose that threat.

Bursts, outbursts and giant flares

Magnetars are erratic. Their activity comes in three broad sizes.

Short bursts. The most common events last around a tenth of a second and release roughly 10^30 to 10^34 joules, mostly as soft gamma rays and hard X-rays. They often come in storms of dozens or hundreds within days.

Outbursts. Over weeks to months, a magnetar's steady X-ray brightness can rise by a factor of 10 to 1,000, then fade. Several magnetars were discovered only because an outburst made them suddenly bright.

Giant flares. The rarest and most violent events. A giant flare begins with an initial spike lasting a fraction of a second, followed by a pulsating tail lasting a few minutes that is modulated by the star's spin.

The mechanism: starquakes and magnetic reconnection

A neutron star has a solid crust — a crystal lattice of nuclei — about a kilometre thick. As the magnetar's interior field slowly evolves, it stresses the crust. When the stress exceeds what the crust can bear, it cracks, a starquake, and the field lines anchored in the crust are suddenly rearranged. The outer magnetic field, now twisted, snaps into a lower-energy configuration through magnetic reconnection, the same process that powers solar flares but scaled up enormously. The energy released heats a fireball of electron–positron pairs and photons that is trapped by the field, producing the pulsating tail.

The three galactic giant flares

Only three giant flares have been recorded from magnetars in or near our galaxy:

Date	Source	Location	Notes
5 March 1979	SGR 0526-66	Large Magellanic Cloud	First giant flare detected; showed an 8-second periodicity
27 August 1998	SGR 1900+14	Milky Way	Disturbed Earth's ionosphere on the night side
27 December 2004	SGR 1806-20	Milky Way, beyond the galactic centre	~100 times more energetic than the other two

The 2004 event stands apart. Analysing data from several spacecraft, Kevin Hurley and colleagues reported in Nature (2005) a total isotropic energy of about 2 × 10^46 erg, or 2 × 10^39 joules (for an assumed distance of 15 kiloparsecs; estimates vary with the uncertain distance) — roughly what the Sun emits in about 150,000 years (our calculation using the Sun's luminosity of 3.8 × 10^26 watts). The flare was the brightest extrasolar transient ever recorded, briefly brighter in gamma rays than the full Moon in visible light, and it measurably disturbed Earth's upper atmosphere. Distance estimates for SGR 1806-20 range between about 30,000 and 50,000 light-years. The Hurley team noted that, seen from a great distance, a flare like this would look like a short gamma-ray burst, implying that some short GRBs seen from other galaxies may actually be magnetar giant flares. One such candidate is GRB 200415A, traced to the nearby Sculptor Galaxy (NGC 253) in 2020.

The fast radio burst connection

Fast radio bursts (FRBs) are millisecond-long flashes of radio waves, first identified in 2007, that come from other galaxies and carry so much energy that their sources were a mystery for over a decade. Our explainer on fast radio bursts covers the wider story; magnetars have become the leading explanation for at least some of them.

The turning point came on 28 April 2020. The magnetar SGR 1935+2154, in our own galaxy, had entered an active phase with a storm of X-ray bursts. That day:

• Canada's CHIME radio telescope detected a bright radio burst in the 400–800 MHz band from its direction (its fluence, a few hundred kilojansky-milliseconds, is uncertain because the burst arrived through a side lobe of the telescope).
• The STARE2 array in the United States, designed to catch bright galactic radio bursts, detected the same event at 1.28–1.47 GHz with a fluence of 1.5 mega-jansky-milliseconds (Bochenek et al., Nature, November 2020).
• Space-based X-ray instruments recorded a simultaneous X-ray burst from the magnetar.

The burst, named FRB 200428, was about 4,000 times more energetic than any previously observed Galactic radio burst on similar timescales and only about 40 times weaker than the faintest extragalactic FRB then known. Three papers in Nature in November 2020 concluded that a magnetar can produce FRB-like emission.

That did not close the case. FRB 200428 was weaker than typical cosmic FRBs, and most magnetar X-ray bursts are not accompanied by radio bursts — China's FAST telescope monitored SGR 1935+2154 during many X-ray bursts in the same period and detected only weak radio pulses. Some repeating FRBs also live in environments, such as old globular clusters, where a young magnetar would be unexpected. The current consensus is that magnetars produce at least some FRBs, with other sources possible.

Finding magnetars: SGRs and AXPs

Magnetars were discovered twice, under two names, before astronomers realised they were the same kind of object.

Soft gamma repeaters (SGRs) were first noticed through their repeated bursts of soft gamma rays. They were initially confused with gamma-ray bursts until the repetition showed they were a distinct class.

Anomalous X-ray pulsars (AXPs) were found as slowly spinning X-ray pulsars that emitted more X-ray energy than their spin-down could explain, and that had no companion star to feed them.

In 2002, bursts were detected from AXPs, and the two classes were unified as magnetars. Today, they are found by X-ray and gamma-ray observatories such as NASA's Swift, NASA's Fermi Gamma-ray Space Telescope, NICER on the International Space Station, ESA's XMM-Newton and INTEGRAL, and NuSTAR. Radio follow-up has added a few magnetars that also emit pulsed radio waves.

The McGill Online Magnetar Catalog

The standard reference list is the McGill Online Magnetar Catalog, maintained at McGill University and described by Olausen and Kaspi in The Astrophysical Journal Supplement Series (2014). As of its last update on 17 November 2020 it lists 30 objects:

Class	Confirmed	Candidates	Total
Soft gamma repeaters	12	4	16
Anomalous X-ray pulsars	12	2	14
All	24	6	30

A few notable entries from the catalogue:

Magnetar	Spin period	Estimated surface field
SGR 1806-20	~7.55 s	~2 × 10^15 G (2 × 10^11 T)
SGR 1935+2154	~3.25 s	~2.2 × 10^14 G
1E 1547.0-5408	~2.07 s	~3.2 × 10^14 G
Swift J1818.0-1607	~1.36 s	~3.5 × 10^14 G
1E 1841-045	~11.8 s	~7 × 10^14 G (slowest spin in the list)

The catalogue also flags borderline objects. PSR J1846-0258, spinning every 0.33 seconds, is usually classed as a rotation-powered pulsar, but it produced a magnetar-like outburst in 2006 — evidence that the line between pulsars and magnetars is blurry.

How many magnetars are there?

Only a few dozen magnetars are known, but that number is shaped by selection effects. Magnetars stay active for roughly 10,000 years before their fields decay enough to quiet them. Many are faint between outbursts and are found only when they flare. Some astronomers estimate that a significant fraction of all neutron stars pass through a magnetar phase, which would mean that a large population of "retired" magnetars exists in the Milky Way, indistinguishable today from ordinary, quiet neutron stars. Our galaxy is thought to contain hundreds of millions of neutron stars in total.

New magnetars keep turning up. Swift J1818.0-1607, the fastest-spinning entry in the McGill list, was discovered in March 2020 when NASA's Swift observatory caught it bursting. Its spin period and slow-down rate imply an age of only a few hundred years, which would make it one of the youngest neutron stars known in the Milky Way. Discoveries like this suggest that the known sample is a small, recently active slice of a much larger population, and that every new X-ray sky survey is likely to add more.

The ages also tell a story. Many magnetars sit near young supernova remnants or in clusters of massive stars. SGR 1806-20, for example, lies in a cluster of very massive stars, suggesting magnetars come from stars at the upper end of the neutron-star-forming mass range.

Common misconceptions

"Magnetars are the most extreme object in the universe." For magnetic fields, they are. For gravity and density, black holes are more extreme. The title is shorthand for "the most extreme magnetic object."

"A magnetar could wipe out life on Earth from anywhere in the galaxy." No. The 2004 flare came from tens of thousands of light-years away and only disturbed the upper atmosphere. A dangerous flare would require a magnetar within a few light-years, and none is known.

"Magnetars spin fast because they are neutron stars." They spin slowly compared with most pulsars, because their fields brake them strongly.

"All fast radio bursts come from magnetars." Magnetars produce at least some, but the evidence does not yet show they produce all of them.

Frequently asked questions

Q: What is the difference between a magnetar and a pulsar? A: Both are neutron stars. A pulsar shines mainly by converting its rotational energy into radiation. A magnetar's emission is powered mainly by the decay of its enormous magnetic field, which is typically 100 to 1,000 times stronger than a normal pulsar's.

Q: How strong is a magnetar's magnetic field? A: Typically 10^10 to 10^11 tesla at the surface. That is about a quadrillion times Earth's field and billions of times stronger than the strongest steady laboratory magnets, which reach about 45 tesla.

Q: Could a magnetar destroy Earth? A: Not from any known distance. Its magnetic field would only be lethal within about 1,000 km. A giant flare could damage the ozone layer only if the magnetar were within a few light-years, and no magnetar is known to be that close.

Q: Do magnetars cause fast radio bursts? A: At least some. In April 2020 the Milky Way magnetar SGR 1935+2154 produced FRB 200428, detected by CHIME and STARE2 along with a simultaneous X-ray burst. Other FRB sources may also exist.

Q: How many magnetars are known? A: The McGill Online Magnetar Catalog lists 30 objects, 24 confirmed and 6 candidates, as of its November 2020 update. A few more have been reported since.

Q: How long does a magnetar last? A: Its active phase is thought to last about 10,000 years. After that the field has decayed enough that it behaves like an ordinary, quiet neutron star, though it still exists.

Q: Could a magnetar become a black hole? A: Only if it gained enough mass to exceed the maximum neutron-star mass, thought to be around 2–3 solar masses. Isolated magnetars have no source of extra mass, so this is not expected for the known population.

The bottom line

Magnetars are neutron stars whose magnetic fields, at 10^10–10^11 tesla, dominate their behaviour. They are young, spin slowly, and shine by burning through their own magnetic energy in bursts, outbursts and, rarely, giant flares like the one SGR 1806-20 released in 2004. They push quantum electrodynamics into regimes no laboratory can reach, and since 2020 they have offered the best explanation for at least some fast radio bursts. Only a few dozen are catalogued, but each one is a natural laboratory for the strongest magnetism in the known universe.

Sources

• McGill Online Magnetar Catalog (Olausen & Kaspi 2014; last updated 17 November 2020): https://www.physics.mcgill.ca/~pulsar/magnetar/main.html
• Hurley et al., giant flare from SGR 1806-20, Nature 434 (2005), preprint: https://arxiv.org/abs/astro-ph/0502329
• Bochenek et al., "A fast radio burst associated with a Galactic magnetar," Nature 587, 59–62 (2020): https://arxiv.org/abs/2005.10828
• Robert C. Duncan, "Magnetars, Soft Gamma Repeaters and Very Strong Magnetic Fields" (University of Texas, archived): https://webarchive.library.unt.edu/web/20081015051433mp_/http://solomon.as.utexas.edu/~duncan/magnetar.html

Related Reading

• Neutron Stars: The Most Extreme Objects in the Universe
• What Is a Supernova — and Why They Matter to Life on Earth
• What Happens When a Star Dies? Supernovas, White Dwarfs, and Black Holes Explained
• What Are Quasars — The Brightest Objects in the Universe?
• What Happens Inside a Black Hole — Beyond the Event Horizon