A magnetar is a type of neutron star with an extraordinarily powerful magnetic field, typically in the range of 1014 to 1015 gauss. That is roughly a quadrillion times stronger than Earth’s magnetic field and hundreds of times stronger than the fields of ordinary neutron stars. These objects reveal themselves through bursts of X-rays and gamma rays, and they have become one of the most studied extreme environments in modern astrophysics, partly because the physics near their surfaces pushes into regimes where even empty space behaves strangely.
How Magnetars Were Identified
Magnetars were not discovered all at once. Throughout the 1980s and 1990s, astronomers noticed two categories of neutron stars that did not fit neatly into the standard picture of rotation-powered pulsars. One category, soft gamma repeaters (SGRs), produced repeated bursts of soft gamma rays. The other, anomalous X-ray pulsars (AXPs), emitted steady but puzzlingly bright X-rays without an obvious energy source like a companion star or rapid rotation. In the late 1990s, Robert Duncan and Christopher Thompson proposed that both types could be explained by a single idea: neutron stars carrying magnetic fields of several times 1014 gauss. X-ray observations confirmed that a substantial fraction of newly born neutron stars carry fields this strong, and these magnetars could account for “missing” radio pulsars in young supernova remnants where astronomers expected to find them but didn’t.1PubMed Central. Magnetars Spectral analysis of archival data for several known AXPs and SGRs supported the magnetar model, showing that the X-ray emission could be explained as thermal radiation from a surface bathed in fields of 1014 to 1015 gauss.2The Astrophysical Journal. Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters: Spectral Fits and the Magnetar Model
Where the Magnetic Field Comes From
The leading explanation involves what happens in the first seconds after a massive star collapses into a neutron star. The newborn object, called a protoneutron star, is extremely hot and convective, with material churning vigorously in its interior. If the protoneutron star is rotating fast enough, that convection can drive a dynamo, the same basic mechanism that generates Earth’s magnetic field but operating under far more extreme conditions. Three-dimensional simulations of this process show that sufficiently rapid rotation can produce dipolar magnetic fields on magnetar scales. The dynamo saturates in a regime where magnetic energy exceeds the kinetic energy of the convective motions by up to a factor of ten.3PubMed Central. Magnetar formation through a convective dynamo in protoneutron stars
Not every collapsing star produces a magnetar. The initial rotation rate is critical, and the path to rapid core rotation may depend on the star’s history before collapse. One intriguing line of evidence comes from the star cluster Westerlund 1, which hosts a magnetar whose progenitor likely had a mass of at least 40 solar masses. Analysis of a nearby runaway star suggests the magnetar formed in a close binary system, where mass exchange between the two stars spun up the core of the future magnetar progenitor and prevented it from slowing down before the supernova. This points to binary interactions as a potentially important ingredient in making at least some magnetars.4Astronomy & Astrophysics. A VLT/FLAMES survey for massive binaries in Westerlund 1. IV. Wd1-5 – binary product and a pre-supernova companion for the magnetar CXOU J1647-45?
Starquakes and Crust Fracture
A magnetar’s outer layer is a crystalline solid, a crust made of neutron-rich nuclei arranged in a lattice, somewhat like the crust of a rocky planet but immensely denser. Beneath this rigid shell, the magnetic field is not static. It evolves over time, and as it shifts, it exerts changing forces on the crust. The crust responds by deforming elastically, storing energy like a bent spring. Eventually the accumulated stress exceeds the breaking point, and the crust fractures.
Detailed calculations of this process show that for a magnetar with a field around 1015 gauss, the time to reach fracture can be as short as a few years. For an ordinary radio pulsar with a much weaker field, the same process would take more than a million years. The elastic energy stored in the crust before it breaks ranges from about 1041 to 1045 ergs depending on how the energy is distributed spatially, with the deepest parts of the crust generally holding the most. A fracture in the shallow crust typically releases around 1041 ergs, comparable to the energy seen in magnetar outbursts.5The Astrophysical Journal. Accumulation of Elastic Strain toward Crustal Fracture in Magnetized Neutron Stars
The picture is not entirely clean, though. Some work shows that in certain magnetic field configurations, the strongly sheared field near a crack creates a magnetic back-reaction that prevents the crack surfaces from moving apart freely. Instead of a sudden rupture, the crust undergoes something closer to plastic flow, with the crack evolving on a slow timescale rather than releasing a burst of energy all at once.6Monthly Notices of the Royal Astronomical Society. On the dynamics of mechanical failures in magnetized neutron star crusts Whether a given event produces a sharp burst or a more gradual release probably depends on the local geometry of the field and the crust.
Giant Flares
The most spectacular events associated with magnetars are giant flares, blasts of gamma rays so intense they can briefly outshine everything else in the gamma-ray sky. Only three have been firmly detected from sources in our own galaxy or its immediate neighborhood. The most powerful on record came from SGR 1806−20 on December 27, 2004, and it was bright enough to measurably disturb Earth’s ionosphere from roughly 50,000 light-years away.
The physical picture for how giant flares unfold draws on ideas originally developed for solar flares. A crustal fracture or large-scale rearrangement of the magnetic field launches energy into the magnetosphere, where the field lines twist and eventually reconnect in a process that converts magnetic energy into heat and particle acceleration. Recent relativistic simulations capture this sequence in detail: magnetic reconnection in the current sheet above the star drives a hot trailing outflow that can power the initial spike of a giant flare, while a thermally layered fireball trapped in the inner magnetosphere produces the pulsating, slowly decaying tail observed afterward.7The Astrophysical Journal Letters. Relativistic Magnetohydrodynamic Simulations of Giant Magnetar Bursts Analysis of the tearing instability in the magnetar’s relativistic current sheet shows that the minimum growth time of the fastest tearing mode comes out to roughly a fraction of a millisecond for typical magnetar parameters, matching the observed sub-millisecond rise times of giant flares.8Monthly Notices of the Royal Astronomical Society. The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanism
When a Giant Flare Hits Earth’s Atmosphere
The 2004 flare from SGR 1806−20 gave researchers a rare chance to measure how a distant astrophysical event can physically alter conditions on Earth. The burst of gamma rays ionized the daytime lower ionosphere, increasing electron densities by a factor of about 100 at roughly 60 kilometers altitude and by a factor of more than 1,000 at about 30 kilometers. These changes were detected as unusually large shifts in the amplitude and phase of very-low-frequency radio signals propagating below the ionosphere. The enhanced conductivity produced by the flare persisted for over an hour, limited by the timescale of chemical processes that gradually neutralized the extra electrons.9Geophysical Research Letters. Massive disturbance of the daytime lower ionosphere by the giant γ‐ray flare from magnetar SGR 1806–20
This was not a dangerous event for people on the ground. Earth’s atmosphere absorbed the radiation effectively, and the effect was confined to a temporary bump in ionization at high altitude. But it demonstrated that a sufficiently powerful magnetar flare from a closer source could, in principle, have more noticeable consequences. The nearest known magnetar is still thousands of light-years away, so this remains a theoretical concern rather than a practical one.
Vacuum Birefringence and Quantum Electrodynamics
One of the most remarkable things about magnetar-strength fields is that they push into a regime where quantum electrodynamics predicts that empty space itself starts behaving like a crystal. In strong enough magnetic fields, the vacuum becomes birefringent, meaning light polarized in different directions travels at slightly different speeds. This effect has been predicted since the 1930s but is vanishingly small in any field achievable in a laboratory. Magnetars offer the only known natural environment where it should be directly observable.
Coordinated X-ray and radio observations of the magnetar 1E 1547.0−5408, using the Imaging X-ray Polarimetry Explorer (IXPE), NICER, and the Parkes radio telescope, have provided the strongest evidence yet. The measurements revealed large polarization degrees in the soft X-ray band, reaching about 65% on average at 2 keV and climbing to nearly 80% at certain rotation phases. Standard models of surface emission without vacuum birefringence struggle to produce polarization this high, but models that include birefringence-governed propagation through the magnetosphere can naturally explain the observations.10PubMed. Vacuum birefringence and the polarized X-ray emission from a radio magnetar This amounts to using a magnetar as a cosmic laboratory for testing a fundamental prediction of quantum field theory that cannot be checked any other way.
How Magnetars Shine in X-Rays
The persistent X-ray emission of magnetars, the glow they produce between bursts, has its own explanation beyond simple surface heat. Currents flowing through the magnetosphere carry charged particles that scatter thermal photons rising from the surface. When a photon’s energy, as seen by the moving charge, matches the cyclotron resonance of the magnetic field, the scattering becomes enormously efficient. This resonant cyclotron scattering boosts and redistributes the thermal photons in both energy and direction, producing a nonthermal tail that extends the spectrum above what a simple hot surface would emit. Systematic fits to X-ray data from ten magnetars showed that this model can account for the soft X-ray emission using the same number of free parameters as the standard empirical approach of fitting a blackbody plus a power law.11The Astrophysical Journal. Resonant Cyclotron Scattering in Magnetars’ Emission
Polarization measurements are now adding another dimension. IXPE observations of the magnetar 1E 1841−045 found that the hard X-ray component is polarized above 65%, consistent with synchrotron or curvature radiation, while the intermediate power-law component shows about 30% polarization, matching predictions for resonant Compton scattering. The soft thermal component is more modestly polarized at about 25%, possibly produced by a condensed surface or a bombarded atmosphere.12The Astrophysical Journal Letters. IXPE Detection of Highly Polarized X-Rays from the Magnetar 1E 1841-045 Even newly discovered magnetars are being caught in their early active phases: IXPE followed up the magnetar EP J223759.5+531421, discovered in mid-2026 by the Einstein Probe, and detected polarization varying dramatically with rotational phase, with the hotter surface component being much more polarized and the polarization angle swinging smoothly through a full 180 degrees per rotation.13arXiv. IXPE observation of the new magnetar source EP J223759.5+531421
Low-Field Magnetars and What They Mean
If magnetars are defined by extreme magnetic fields, what do you do with one that has an ordinary field? The discovery of SGR 0418+5729 posed exactly this problem. It produced bursts indistinguishable from classic magnetar activity, but X-ray timing measurements showed that its surface dipole field could not be greater than about 7.5 × 1012 gauss, squarely within the range of normal radio pulsars.14PubMed. A low-magnetic-field soft gamma repeater A second low-field magnetar, Swift J1822.3−1606, was soon identified with similar properties. Together, these discoveries showed that a high surface dipole field is not strictly necessary for magnetar-like behavior.15International Journal of Modern Physics D. Low-Magnetic-Field Magnetars
The resolution probably lies in the difference between the large-scale dipole field, which is what spin-down timing measures, and smaller-scale or internal fields that can be much stronger. Three-dimensional simulations of magnetic field evolution in magnetar crusts show that even when the global-scale field is shared between poloidal and toroidal components, magnetic instabilities transfer energy into non-axisymmetric, kilometer-sized features where the local field strength can greatly exceed the global average. These intense small-scale structures can still fracture the crust locally and drive bursts, even if the overall dipole appears modest.16PubMed Central. Magnetic field evolution in magnetar crusts through three-dimensional simulations In other words, what matters for magnetar activity is not just the field you can measure from the outside but the field hidden inside the star.
What Magnetars Become
Magnetars are not permanent. Their extreme magnetic fields decay over time through resistive processes in the crust and core, and as the field weakens, the star’s magnetar activity fades. The timescale for this decay is thought to be on the order of thousands to tens of thousands of years, making magnetars a transient phase rather than a permanent identity. Population modeling of magnetar spin evolution suggests that aging magnetars are likely linked to the population of X-ray dim isolated neutron stars (XDINSs), quietly cooling objects that no longer burst but still emit faint thermal X-rays. The connection to rotating radio transients (RRATs), which were once speculated to be retired magnetars, appears less likely.17Monthly Notices of the Royal Astronomical Society. Modelling spin evolution of magnetars
Powering Superluminous Supernovae
Magnetars have become central to one of the more dramatic puzzles in extragalactic astronomy. Superluminous supernovae, explosions that can be tens of times brighter than ordinary supernovae, need an extra engine beyond the radioactive decay that powers typical events. A rapidly spinning, highly magnetized neutron star left behind by the explosion can do the job: as the magnetar spins down, it pours rotational energy into the expanding debris, keeping it lit far longer and more brightly than it otherwise would be.
Recent observations have strengthened this picture considerably. One superluminous supernova showed undulations in its light curve that could be explained by a tilted accretion disk around the central magnetar undergoing Lense-Thirring precession, a frame-dragging effect predicted by general relativity. The overall brightness and the frequency of the bumps independently constrained the magnetar’s spin period and field strength in a self-consistent way, providing what the authors describe as the first observational evidence of the Lense-Thirring effect in a magnetar environment.18arXiv. Lense-Thirring precessing magnetar engine drives a superluminous supernova
Searching for Magnetar Flares in Other Galaxies
Because giant flares are so luminous, they can in principle be detected from magnetars in other galaxies, but they look almost identical to short gamma-ray bursts at those distances. Disentangling the two populations is tricky. A refined search of archival data from the Fermi Gamma-ray Burst Monitor, guided by observational criteria tuned to the known properties of magnetar flares, turned up four additional extragalactic giant flare candidates beyond those previously identified.19Astronomy & Astrophysics. From rare events to a population: Discovering overlooked extragalactic magnetar giant flare candidates in archival Fermi Gamma-ray Burst Monitor data Building up this extragalactic sample matters for estimating how common magnetars are across the universe and how often they produce their most extreme outbursts. It also has practical consequences for the study of short gamma-ray bursts themselves: if some fraction of events catalogued as short bursts are actually magnetar flares in nearby galaxies, the true rate and energy distribution of merger-driven bursts needs to be adjusted accordingly.
Quasi-Periodic Oscillations and What They Reveal About Neutron Star Interiors
When a magnetar produces a giant flare, the tail of the event sometimes contains quasi-periodic oscillations, rapid flickering at specific frequencies. These are thought to be seismic vibrations of the neutron star itself, ringing like a bell after the crust fractures. Studying these oscillations is one of the few ways to probe the deep interior of a neutron star, a region where matter exists at densities beyond what can be recreated in any laboratory. Models that incorporate the magnetic field, the elastic crust, and the likely superfluid state of the interior show that the oscillation spectrum depends sensitively on the internal composition, including gradients in the mix of particles and the degree of entrainment between superfluid and normal components.20Monthly Notices of the Royal Astronomical Society. Quasi-periodic oscillations in superfluid magnetars Matching observed frequencies to theoretical predictions is still an imperfect science, but every new flare with detectable oscillations adds constraints on what the inside of a neutron star actually looks like.