Magnetars: When a Neutron Star Becomes a Magnetic Bomb

What is a magnetar?

Among all the objects in the Universe, magnetars are undoubtedly some of the most violent. They are neutron stars with a magnetic field so intense that it surpasses that of any other star by several orders of magnitude. A magnetar can reach \(10^{11}\) teslas, about 1.5 quadrillion times the Earth's magnetic field (25 to 65 microteslas, or 0.25 to 0.65 gauss).

At such extreme intensities, the magnetic field is no longer just a physical property. It alters the structure of atomic orbitals, polarizes the quantum vacuum, and, according to quantum electrodynamics, can give the vacuum birefringence, allowing it to deflect light according to its polarization. At such energy levels, the very stability of matter is called into question: atoms stretch into thin cylinders aligned along the field lines, losing all resemblance to their usual configuration.

How is a magnetar born?

The collapse of a massive star

The birth of a magnetar is closely linked to that of neutron stars. When a massive star, with an initial mass of about eight to twenty solar masses, exhausts its nuclear fuel, its core collapses in a fraction of a second. Matter is compressed to extreme densities: a single teaspoon of this matter could weigh nearly a billion tons. From this collapse, a neutron star is born, an object about 20 kilometers in diameter but containing more mass than the Sun.

Why do some neutron stars become magnetars?

However, not all neutron stars become magnetars. A question remains: why do some develop such an intense magnetic field? Two main mechanisms are currently proposed by astrophysicists.

The dynamo effect mechanism

The first relies on the dynamo effect. At the moment of collapse, if the progenitor star's core spins fast enough, the merging matter enters a regime of intense turbulent convection. These movements of conductive matter, combined with rapid rotation, exponentially amplify the initial magnetic field within tens of milliseconds. Simulations conducted by Robert Duncan and Christopher Thompson since 1992 have shown that this process, under certain initial rotation conditions, can generate magnetic fields on the order of \(10^{11}\) teslas, consistent with observations.

The magnetic legacy of the progenitor star

The second, more recent mechanism suggests that the magnetar's exceptional magnetic field may partly originate from the progenitor star. Some massive stars, called Ap magnetic stars, already possess unusually powerful magnetic fields. The conservation of magnetic flux during the core collapse, combined with the drastic reduction in the object's radius (from hundreds of thousands of kilometers to about twenty), would be enough to amplify this initial field by a factor of about \((R_{\text{star}}/R_{\text{neutron}})^2\), or roughly \(10^{10}\).

Bursts and eruptions: the signature of magnetars

It is through their energetic manifestations that magnetars reveal themselves to our instruments. Several types of events are distinguished.

Soft gamma repeaters (SGRs) are brief, recurrent emissions of relatively low-intensity X-rays and gamma rays. They indicate sustained magnetic activity, linked to crustal readjustments or deformations of field lines in the magnetosphere.

Giant eruptions are the most spectacular events. Three have been detected to date in our Galaxy or its immediate neighbors. The most famous, which occurred on December 27, 2004, from SGR 1806-20, was powerful enough to partially ionize the Earth's upper atmosphere from a distance of about 50,000 light-years. Bryan Gaensler (1973-) and his collaborators estimated that this eruption released in 0.2 seconds an energy equivalent to that of the Sun over 250,000 years.

Finally, some magnetars have been associated with fast radio bursts. In 2020, the detection of FRB 200428 from the magnetar SGR 1935+2154, located in our own Galaxy, provided the first direct evidence that magnetars can produce such bursts.

Comparison of neutron stars: pulsars, magnetars, and isolated X-ray stars

Neutron stars come in several forms depending on their magnetic field and rotational activity. The following table compares their main properties.

N.B.: SGRs and AXPs are now considered two observational manifestations of the same category of objects: magnetars. The historical distinction reflects the initial detection method rather than a fundamental physical difference. Active lifespans are very short on a cosmic scale, as the magnetic field dissipates rapidly, slowing the star until its activity ceases.