A rare type of neutron star with extremely strong magnetic fields, causing intense "starquakes"
Neutron stars, already the densest known stellar remnants (except for black holes), can have magnetic fields billions of times stronger than typical stars. Among them stands out a rare class called magnetars, characterized by the strongest magnetic fields observed so far in the Universe, reaching up to 1015 G or even more. These extremely powerful fields can cause unusual, violent phenomena—starquakes, gigantic flares, and gamma-ray bursts that briefly outshine entire galaxies. This article explores the physics of magnetars, their observable features, and the extreme processes driving their eruptions and surface activity.
1. Nature and formation of magnetars
1.1 Birth as a neutron star
A magnetar is essentially a neutron star formed during a core-collapse supernova, when the iron core of a massive star collapses. During collapse, part of the star's core angular momentum and magnetic flux can be compressed to an exceptionally high level. Ordinary neutron stars have fields of 10^9–1012 G, while magnetars can increase these to 1014–1015 G, or possibly even more [1,2].
1.2 Dynamo hypothesis
Extremely strong magnetic fields in magnetars may arise from a dynamo mechanism in the early proto-neutron star phase:
- Rapid rotation: If a newly born neutron star initially spins with a millisecond period, convection and differential rotation can greatly amplify the magnetic field.
- Short-lived dynamo: Such a convective dynamo can operate for seconds or minutes after collapse, setting magnetar-level fields.
- Magnetic braking: Over several thousand years, strong fields significantly slow the star's rotation, leaving a slower spin period than typical radio pulsars [3].
Not all neutron stars become magnetars—only those whose initial spin and core parameters allow extreme field amplification.
1.3 Duration and rarity
Magnetars maintain their extremely strong fields for about 104–105 years. As the star ages, magnetic field decay can cause internal heating and eruptions. Observations show that magnetars are quite rare—only a few dozen such objects have been confirmed or suspected in the Milky Way and nearby galaxies [4].
2. Magnetic field strength and effects
2.1 Magnetic field scales
Magnetar fields exceed 1014 G, while typical neutron star fields reach 109–1012 G. For comparison, Earth's surface magnetic field is about ~0.5 G, and laboratory magnets rarely exceed a few thousand G. Thus, magnetars hold the record for the strongest persistent fields in the Universe.
2.2 Quantum electrodynamics and photon splitting
When fields are \(\gtrsim 10^{13}\) G, quantum electrodynamics (QED) effects become important (e.g., vacuum birefringence, photon splitting). Photon splitting and polarization changes can affect how radiation escapes the magnetar magnetosphere, altering spectral features, especially in X-ray and gamma-ray bands [5].
2.3 Stresses and "starquakes"
Extremely strong internal and crustal magnetic fields can strain the neutron star's crust to breaking. Starquakes—sudden crust fractures—can rearrange magnetic fields and trigger flares or high-energy photon outbursts. Sudden stress release can also slightly change the star's spin rate, leaving detectable spin period "glitches".
3. Observed features of magnetars
3.1 Soft gamma repeaters (SGR)
Even before the term "magnetar" was established, certain soft gamma repeaters (SGR) were known for intermittent gamma or hard X-ray bursts recurring irregularly. These bursts usually last from fractions of a second to several seconds, with average peak brightness. We now understand that SGRs are magnetars in quiescence, occasionally disturbed by "starquakes" or magnetic field rearrangements [6].
3.2 Anomalous X-ray pulsars (AXP)
Another class, anomalous X-ray pulsars (AXP), are neutron stars with spin periods lasting several seconds, but their X-ray brightness is too high to be explained by spin-down alone. Additional energy likely comes from magnetic field decay, powering the X-ray emission. Many AXPs also show bursts resembling SGR episodes, confirming their magnetar nature.
3.3 Giant flares
Magnetars sometimes emit giant flares—especially energetic events whose peak brightness can briefly exceed 1046 erg·s−1. Examples: the 1998 flare from SGR 1900+14 and the 2004 flare from SGR 1806–20, the latter even affecting Earth's ionosphere from 50,000 light-years away. During such flares, a bright initial phase spike is often observed, followed by a chain of pulsations modulated by the star's rotation.
3.4 Spin and spin "glitches"
Like pulsars, magnetars can show periodic pulses according to the spin frequency, but with slower average periods (~2–12 s). Magnetic field decay imposes an additional spin-down torque, so they slow down faster than ordinary pulsars. Occasionally, "glitches" (sudden changes in spin frequency) can occur after crust fractures. By observing these spin changes, we can estimate the internal interaction between the crust and the superfluid core.
4. Magnetic field decay and activity mechanisms
4.1 Heat from field decay
Extremely strong magnetars gradually decay their fields, releasing energy as heat. This internal heating can maintain surface temperatures of hundreds of thousands or millions of kelvin—much higher than typically cooling neutron stars of the same age. Such heating results in persistent X-ray emission.
4.2 Hall drift and ambipolar diffusion in the crust
Nonlinear interactions in the crust and core—Hall drift (interaction between electron flow and magnetic field) and ambipolar diffusion (charged particle motion responding to the field)—can rearrange fields over 103–106 year timescales, powering flares and persistent emission [7].
4.3 Starquakes and magnetic reconnection
Stress caused by field evolution can trigger crust fractures, releasing sudden energy—these are starquakes. Such fractures can rearrange magnetospheric fields, causing reconnection events or major flares. Models compare these processes to solar flares but on a much larger scale. After a flare, recovery can alter spin frequency or the nature of magnetospheric radiation.
5. Magnetar evolution and final stages
5.1 Long-term fading
Over 105–106 year magnetars likely evolve into more typical neutron stars as fields weaken to ~1012 G. Then the star's active phenomena (flares, giant eruptions) become rare. Eventually, such a star cools and its X-ray emission decreases, making it resemble an older "dead" pulsar with only a relatively small residual magnetic field.
5.2 Binary interactions?
Few binary systems with magnetars have been observed, but some such pairs may exist. If a magnetar has a close stellar companion, mass transfer could cause additional flares or alter spin evolution. However, observational "gaps" or the short lifetime of magnetars may explain why very few such binaries are currently known.
5.3 Possible mergers
Theoretically, a magnetar could merge with another neutron star or black hole, emitting gravitational waves and possibly causing a short gamma-ray burst. Such events would likely far exceed typical magnetar flares in terms of released energy. Observationally, this remains speculative, but merging neutron stars with very strong fields would be unique "cosmic laboratories."
6. Importance for astrophysics
6.1 Gamma-ray bursts
Some short or long gamma-ray bursts could be powered by magnetars formed during core collapse or merger events. Rapidly spinning “millisecond magnetars” can release enormous rotational energy, driving or shaping the GRB jet. Observations of some GRB afterglow plateaus match additional energy input from a newly born magnetar.
6.2 Ultraluminous X-ray sources?
Strong B fields can cause powerful outflows or radiation focusing, potentially explaining some ultraluminous X-ray sources (ULX) if accretion occurs onto a neutron star with a magnetar-like field. In such systems, luminosity can exceed the usual Eddington limit, especially if radiation is beamed [8].
6.3 Dense matter and QED studies
Extreme conditions at the magnetar surface allow the study of QED in strong fields. Polarization or spectral line observations can reveal vacuum birefringence or photon splitting—phenomena impossible to reproduce in Earth laboratories. This helps improve nuclear physics and quantum field theories under ultradense conditions.
7. Observation campaigns and future studies
- Swift and NICER: Observing magnetar outbursts in X-ray and gamma bands.
- NuSTAR: Sensitivity to the hard X-ray range, helping to detect high-energy radiation from bursts or giant flares.
- Radio searches: Some magnetars occasionally emit radio pulses, linking magnetars and ordinary pulsars within one population.
- Optical/IR observations: Rare optical or IR counterparts are very faint but can show jets or dust re-radiation after bursts.
Future or planned observatories, such as Europe's ATHENA (X-ray domain), promise even deeper insights: to study weaker magnetars or capture the onset of a giant burst in real time.
8. Conclusion
Magnetars are extreme examples in the physics of neutron stars. Their incredible magnetic fields, reaching 1015 G, cause violent eruptions, starquakes, and unstoppable gamma bursts. Formed from the collapse of massive stars under special conditions (rapid rotation, favorable dynamo action), magnetars are short-lived cosmic phenomena, shining brightest over a period of ~104–105 years until field decay reduces their activity.
In terms of observation, soft gamma repeaters and anomalous X-ray pulsars represent magnetars in different states, sometimes emitting impressive giant bursts observable even on Earth. Studies of these objects expand our knowledge of quantum electrodynamics in extremely strong fields, the structure of nuclear matter, and processes that can trigger bursts of neutrinos, gravitational waves, and electromagnetic eruptions. As field decay models improve and magnetar outbursts are observed with increasingly advanced multiwavelength instruments, magnetars will continue to open some of the most exotic corners of astrophysical research—where matter, fields, and fundamental forces converge in astonishing extremes.
References and further reading
- Duncan, R. C., & Thompson, C. (1992). "Formation of very strongly magnetized neutron stars: Implications for gamma-ray bursts." The Astrophysical Journal Letters, 392, L9–L13.
- 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, 255–300.
- Kouveliotou, C., et al. (1998). "An X-ray pulsar with a superstrong magnetic field in the soft gamma-ray repeater SGR 1806-20." Nature, 393, 235–237.
- Mereghetti, S. (2008). "The strongest cosmic magnets: Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars." Astronomy & Astrophysics Review, 15, 225–287.
- Harding, A. K., & Lai, D. (2006). "Physics of strongly magnetized neutron stars." Reports on Progress in Physics, 69, 2631–2708.
- Kaspi, V. M., & Beloborodov, A. M. (2017). "Magnetars." Annual Review of Astronomy and Astrophysics, 55, 261–301.
- Pons, J. A., et al. (2009). "Magnetic field evolution in neutron star crusts." Physical Review Letters, 102, 191102.
- Bachetti, M., et al. (2014). "An ultraluminous X-ray source powered by an accreting neutron star." Nature, 514, 202–204.
- Woods, P. M., & Thompson, C. (2006). "Soft gamma repeaters and anomalous X-ray pulsars: Magnetar candidates." Compact Stellar X-ray Sources, Cambridge University Press, 547–586.