What Is a Magnetar? The Strongest Magnets in the Universe
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A magnetar is a neutron star with a magnetic field a thousand times stronger than an ordinary one and quadrillions of times stronger than the Earth's. At those strengths the field itself dominates the object's behaviour, and the physics stops resembling anything that can be reproduced anywhere.
How strong is strong
Magnetic field strength is conventionally given in tesla, with a fridge magnet around a hundredth of one, a hospital scanner around one and a half to three, and the strongest sustained laboratory fields around forty-five. A typical neutron star reaches around a hundred million. A magnetar reaches ten to a hundred billion tesla. Those numbers are difficult to make meaningful, and the physical consequences help. At such field strengths the vacuum itself becomes birefringent, meaning empty space behaves like a crystal and splits light into two polarisations, an effect predicted by quantum electrodynamics and detectable only in these conditions. Atoms are distorted into thin cylinders aligned with the field. The field energy alone exceeds the energy that would be released by converting the sun entirely into energy. At a distance of a thousand kilometres such an object would erase the magnetic strip on a credit card, and considerably closer it would disrupt the electrical activity of a nervous system.
What they do
Their observable behaviour is dominated by the field rather than by rotation, which distinguishes them from ordinary pulsars:
- •Slow rotation, typically once every two to twelve seconds, and rapid slowing, because the intense field brakes the star quickly
- •X-ray emission powered by the decay of the magnetic field rather than by rotation or by accretion, which is the defining energy source
- •Bursts, with sudden releases of X-rays and gamma rays lasting fractions of a second, occurring irregularly and in some cases in storms
- •Giant flares, extremely rare and extraordinarily powerful, with one in 2004 from a magnetar in our own galaxy briefly outshining everything else in gamma rays and measurably disturbing the Earth's upper atmosphere from fifty thousand light years away
- •Starquakes, in which the rigid crust fractures under magnetic stress, which is the leading explanation for the bursts
- •Short active lifetimes, since the field decays over roughly ten thousand years, after which the object becomes a quiet neutron star, meaning most neutron stars that were magnetars no longer behave as one
Where the field comes from
The origin is not settled and two mechanisms are proposed. Flux conservation holds that the field of the original star is compressed as the core collapses, and since field strength scales with the inverse square of the radius, an ordinary stellar field becomes enormous when a body the size of the sun collapses to twenty kilometres across, which explains ordinary neutron star fields and struggles to reach magnetar strengths. A dynamo mechanism holds that if the newly formed neutron star is spinning extremely fast, perhaps once every few milliseconds, convection in its interior combined with that rotation amplifies the field enormously during the first seconds of its existence, which can reach the required strength and requires a specific and uncommon set of initial conditions. The dynamo account is generally favoured and predicts that magnetars should be rare, which matches observation, with only a few dozen known. A remaining puzzle is whether magnetars and ordinary pulsars form a continuum or genuinely distinct populations.
What they might explain
These objects have become candidates for several unexplained phenomena. Fast radio bursts, millisecond pulses of radio energy arriving from cosmological distances and unexplained since their discovery in 2007, were convincingly connected to magnetars in 2020 when a burst of exactly that character was detected from a known magnetar within our own galaxy, which established that at least some such bursts have this origin even if others may not. Some unusually luminous supernovae have been explained by energy injected into the expanding debris by a rapidly spinning newly formed magnetar. Long gamma-ray bursts have been proposed to involve them. They are also relevant to fundamental physics, since the conditions cannot be produced experimentally and observations therefore test predictions about matter and vacuum in extreme fields that no laboratory can reach, which is an unusual position where astronomy supplies data that physics cannot obtain any other way.
The takeaway
Fields of ten to a hundred billion tesla make the vacuum itself split light and distort atoms into cylinders, and the energy stored in the field alone exceeds what converting the sun entirely into energy would release. They rotate slowly, brake fast, and are powered by field decay rather than rotation. A 2004 flare disturbed Earth's atmosphere from fifty thousand light years away. Fast radio bursts were linked to them in 2020.