Imagine an object the size of a city with the mass of the Sun, spinning several times per second, wrapped in a magnetic field a million billion times stronger than a refrigerator magnet. At the surface, the magnetic field would be strong enough to distort the electron clouds around atoms, fundamentally changing the chemistry of matter. From 1,000 kilometers away (roughly the distance between New York and Chicago), the field would be powerful enough to destroy the iron in your blood.
This is a magnetar: the most extreme form of neutron star known, and one of the most powerful objects in the universe. In the 1998 event known as the “magnetar giant flare,” a single burst from SGR 1900+14 lasting approximately 5 minutes released more energy than the Sun emits in 100,000 years. If that magnetar had been located within about 10 light-years of Earth, it would have ended life on our planet.
What a Magnetar Is
Magnetars are a type of neutron star (compact remnants left when massive stars between about 8 and 20 solar masses exhaust their fuel and explode as core-collapse supernovae. When the iron core collapses, electrons and protons merge via inverse beta decay into neutrons, releasing a pulse of neutrinos and leaving a sphere approximately 20 kilometers in diameter with a mass of about 1.4 solar masses. The density is comparable to atomic nuclei: a teaspoon of neutron star material would weigh billions of tons.
What distinguishes magnetars from ordinary neutron stars is their magnetic field. While a typical neutron star has a surface magnetic field of around 10⁸ to 10¹² Gauss (already billions of times stronger than any magnet on Earth), a magnetar’s field reaches 10¹⁴ to 10¹⁵ Gauss (roughly a quadrillion times stronger than Earth’s magnetic field).
The origin of this extraordinary field is not fully understood. Leading theories invoke a convective dynamo mechanism during the first seconds after the neutron star forms, when the proto-neutron star is hot, turbulent, and rapidly rotating. If the rotation is fast enough and the convective flows are sufficiently vigorous during this brief window, they can amplify the seed magnetic field by many orders of magnitude. Neutron stars born rotating fastest (with periods of milliseconds immediately after formation) may be most likely to develop magnetar-strength fields through this process.
Magnetars are observed primarily as two types of objects: Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). Both are now understood to be manifestations of the same underlying magnetar physics, distinguished mainly by their observational properties.
The Magnetic Field That Breaks Physics
At 10¹⁵ Gauss, a magnetar’s field exceeds what physicists call the quantum critical field (approximately 4.4 × 10¹³ Gauss), above which quantum electrodynamics effects become important. In this regime, the vacuum of space itself becomes birefringent (the magnetic field polarizes the vacuum, causing light to travel at slightly different speeds depending on its polarization direction). This is called vacuum birefringence and is a prediction of quantum electrodynamics that has been sought in astronomical observations.
In 2016, a European Southern Observatory team reported observing linear polarization of optical light from the isolated neutron star RX J1856.5-3754 consistent with vacuum birefringence.
At the surface of a magnetar, the field distorts atomic structure dramatically. The electron clouds around atoms, normally roughly spherical, become elongated into thin cylinders aligned with the field lines. This changes the opacity, thermal conductivity, and emission properties of matter in ways that affect the X-ray spectrum we observe from magnetars.
Energy Source: Magnetic Field Decay
Unlike ordinary pulsars, which are powered by rotational energy that gradually spins them down, magnetars derive most of their energy from the decay of their own magnetic fields. The enormous magnetic energy stored in the field is gradually released as:
– Persistent X-ray emission: Magnetars are typically bright X-ray sources even between outbursts, with luminosities of 10³³ to 10³⁵ ergs per second (orders of magnitude more X-ray bright than ordinary neutron stars of comparable age). – Outbursts and flares: Sudden reconfigurations of the magnetic field (“starquakes”) produce short-duration X-ray and gamma-ray bursts, observable across the galaxy and in extreme cases across the universe. – Giant flares: Rarely, magnetars produce catastrophic events releasing 10⁴⁴ to 10⁴⁷ ergs in tenths of a second to minutes (the most energetic events known to occur in the Milky Way other than supernovae).
Because magnetars are powered by field decay rather than rotation, they spin down faster than ordinary neutron stars. A magnetar’s rotation period typically ranges from about 2 to 12 seconds (slow for a neutron star). Most magnetars also have lifetimes of only about 10,000 years as active magnetars before their fields decay to ordinary neutron star levels. There are estimated to be roughly 30 million inactive magnetar remnants in the Milky Way (former magnetars now masquerading as ordinary neutron stars).
The 1998 and 2004 Giant Flares
Giant flares are the most spectacular magnetar events. Only three have been observed in the modern era (two from objects in the Milky Way’s vicinity).
August 27, 1998: SGR 1900+14, located about 20,000 light-years away, produced a giant flare that lasted approximately 5 minutes. The initial hard gamma-ray spike was followed by a pulsating soft gamma-ray tail modulated by the neutron star’s 5.16-second rotation. The peak flux briefly saturated virtually every gamma-ray satellite in Earth orbit. It was the second-most energetic event recorded from beyond the solar system up to that point.
December 27, 2004: SGR 1806-20 produced the most energetic giant flare yet observed from within the Milky Way. The initial spike lasted 0.2 seconds but released approximately 10⁴⁶ ergs (more energy than the Sun emits in 250,000 years). Had SGR 1806-20 been located within about 10 light-years of Earth rather than its actual ~50,000 light-years, the gamma-ray dose would have been comparable to a nearby supernova. The December 27, 2004 flare was so intense that it partially ionized Earth’s upper atmosphere from 50,000 light-years away.
In 2020, a giant flare detected from the Sculptor Galaxy (NGC 253) at 11.4 million light-years was the first magnetar giant flare observed from beyond the Magellanic Clouds. Its characteristics closely matched those of the 2004 event, providing independent confirmation of the physical model.
Magnetars and Fast Radio Bursts
One of the most exciting developments in magnetar astrophysics in recent years has been the connection to Fast Radio Bursts (FRBs) (millisecond-duration pulses of radio emission first discovered in 2007 from extragalactic sources). The origin of FRBs remained mysterious for over a decade: their dispersion measures indicated extragalactic origins, but no source had been directly pinpointed.
In April 2020, a magnetar in the Milky Way (SGR 1935+2154) was observed to produce a burst of radio waves simultaneously with an X-ray flare. The radio burst had properties (dispersion, luminosity) consistent with a less-luminous version of an extragalactic FRB. This was the first direct observational evidence linking a known magnetar outburst to an FRB-like event, providing strong support for the hypothesis that at least a significant fraction of FRBs are produced by magnetar activity.
This does not mean all FRBs are magnetars (the diverse properties of observed FRBs suggest multiple possible sources), but the SGR 1935+2154 event established that magnetars can produce radio bursts of the relevant energy and timescale. It opened a new chapter in magnetar astrophysics and FRB research.
Observing Magnetars
Magnetars are observable primarily at X-ray and gamma-ray wavelengths, accessible only from space-based telescopes. Key observatories have included: – RXTE (Rossi X-ray Timing Explorer): Crucial for early characterization of SGRs and AXPs in the 1990s–2000s. – XMM-Newton and Chandra: Detailed spectral and timing studies. – NuSTAR: Hard X-ray observations sensitive to the high-energy emission from magnetar flares. – INTEGRAL and Fermi: Critical for detecting giant flares and linking magnetar outbursts to hard gamma-ray emission.
Currently, roughly 30 confirmed or candidate magnetars are known in the Milky Way and nearby galaxies, a number expected to grow as survey capabilities improve.
What is a magnetar?
A magnetar is a type of neutron star with an exceptionally powerful magnetic field (roughly 10¹⁴ to 10¹⁵ Gauss, or about a quadrillion times stronger than Earth’s magnetic field). Magnetars form from the supernova explosions of massive stars and are about 20 kilometers in diameter with roughly the mass of the Sun. Unlike ordinary neutron stars (pulsars), which are powered primarily by rotational energy, magnetars are powered by the gradual decay of their own magnetic fields, producing intense X-ray and gamma-ray emission and occasional catastrophic flares.
How strong is a magnetar’s magnetic field?
A magnetar’s surface magnetic field is approximately 10¹⁴ to 10¹⁵ Gauss. For comparison, Earth’s magnetic field is about 0.5 Gauss, a typical refrigerator magnet is about 50 Gauss, a standard MRI machine reaches about 15,000 Gauss, and the strongest electromagnets ever built on Earth reach about 4.5 × 10⁵ Gauss. A magnetar’s field is roughly a billion to ten billion times stronger than any magnet we have built, and exceeds the quantum critical field at which the vacuum itself becomes birefringent.
What is a magnetar giant flare?
A magnetar giant flare is a catastrophic release of magnetic energy from a magnetar’s outer crust, triggered by sudden stress fractures (starquakes) in the neutron star’s rigid crust as the magnetic field evolves. Giant flares release between 10⁴⁴ and 10⁴⁷ ergs in fractions of a second to several minutes (equivalent to hundreds of thousands of years of normal solar output). Only three magnetar giant flares have been recorded from within or near the Milky Way since the modern era of gamma-ray astronomy. They are detectable across millions of light-years.
Could a magnetar harm Earth?
A magnetar giant flare within about 10 light-years of Earth could, in principle, irradiate and partially sterilize the surface by gamma-ray and X-ray exposure. The December 2004 event from SGR 1806-20, located ~50,000 light-years away, partially ionized Earth’s upper atmosphere. No known magnetar is close enough to pose a threat (the nearest confirmed magnetar is over 1,000 light-years away). The probability of a magnetar giant flare within 10 light-years is extremely low given the rarity of such events and the low density of magnetars near the Sun.
How do magnetars relate to fast radio bursts?
In April 2020, the Milky Way magnetar SGR 1935+2154 produced a brief, powerful radio burst simultaneously with an X-ray flare. The radio burst had properties consistent with a dimmer version of the extragalactic fast radio bursts (FRBs) detected since 2007. This was the first direct evidence linking magnetar activity to FRBs, supporting the hypothesis that magnetars (possibly via sudden magnetic reconnection events or other mechanisms) are responsible for at least some of the mysterious millisecond-duration radio flashes detected from galaxies billions of light-years away.
How long does a magnetar remain active?
A magnetar’s intense magnetic activity lasts only about 10,000 years before the field decays to ordinary neutron star levels. After that, the former magnetar continues to spin down and cool as a standard neutron star, essentially indistinguishable from those formed without extreme magnetic fields. It is estimated that roughly 30 million defunct magnetars may exist in the Milky Way (neutron stars that were once magnetars but have since cooled and lost their extreme activity). This means magnetars may represent a significant fraction of the total neutron star population.
Sources
Duncan, R.C., & Thompson, C. (1992). Formation of very strongly magnetized neutron stars: implications for gamma-ray bursts. The Astrophysical Journal, 392, L9–L13. doi:10.1086/186413
Thompson, C., & Duncan, R.C. (1995). The soft gamma repeaters as very strongly magnetized neutron stars (I. Radiative mechanism for outbursts). Monthly Notices of the Royal Astronomical Society, 275(2), 255–300. doi:10.1093/mnras/275.2.255
Hurley, K. et al. (2005). An exceptionally bright flare from SGR 1806-20 and the origins of short-duration γ-ray bursts. Nature, 434(7037), 1098–1103. doi:10.1038/nature03519
CHIME/FRB Collaboration. (2020). A bright millisecond-duration radio transient from a Galactic magnetar. Nature, 587(7832), 54–58. doi:10.1038/s41586-020-2863-y
Mereghetti, S. (2008). The strong-field regime of neutron star physics: magnetars. The Astronomy and Astrophysics Review, 15(4), 225–287. doi:10.1007/s00159-008-0011-z
Turolla, R., Zane, S., & Watts, A.L. (2015). Magnetars: the physics behind observations. A review. Reports on Progress in Physics, 78(11), 116901. doi:10.1088/0034-4885/78/11/116901
Further reading: Magnetar on Wikipedia