A star’s atmosphere is solidified by its magnetic field.

The concepts of neutron star and black hole as a product ofcollapsecollapse of a star under the effect of its own gravitation within the framework of the theory of general relativity date from the end of the 1930s when Robert Oppenheimer laid the foundation of these concepts with articles written in collaboration with his students from the ‘era : ” On Massive Neutron Cores », with Georges Volkoff, and « On Continued Gravitational Contraction with Hartland Snyder.

The revolution on their subject will occur during the 1960s, first from a theoretical point of view with in particular work that we owe to Nobel Prize winners in physics Kip Thorne and Roger Penrose but also to researchers of the caliber of John Wheeler and Stephen Hawking. But, any theory is only valid through the observational tests it undergoes and the first signature of the existence of neutron stars was only found in 1967 with the discovery of the first pulsar. In 1972, it was the hole of the first black hole with observations concerning the source of X-rays baptized Cygnus X1, clearly associated with a compact star and without stellar signature, but causing by its mass the oscillating movements of a supergiantsupergiant forming with him a binary systembinary system.

The rich physics of neutron stars

The study of black holes and neutron stars continues today. Neutron stars are less of a dream for the general public, but they are rich in spectacular physical effects, involving superfluidity as well as gravitational wavesgravitational waves and even what is called the birefringencebirefringence magnetism of the quantum vacuum. The latter can modify the polarization of the light waves. We will come back to this in more detail.

A new example of the existence of a vast field of study still open with regard to neutron stars has just been given with a publication on arXiv. It concerns an exotic form of neutron star that has been discovered more recently and is called a magnetar. As its name suggests, these neutron stars have a particularly intense magnetic field, 100 to 1000 times stronger than those of standard neutron stars which are already spectacularly strong.

These are the data collected by a NASA satellite, Imaging X-ray Polarimetry Explorer (IXPE), which was launched last December, which surprised astrophysicists. They therefore concerned a magnetar, the one known in the catalogs under the name of 4U 0142+61, located in the constellation Cassiopeia, about 13,000 light years from Solar systemSolar system.

The magnetarsmagnetars can be particularly bright in the ray domain with flares that can release in a second an amount of energy millions of times greater than what our Sun emits in a year.

IXPE can measure the polarization of X-rays at various energies. Recall, as can be seen in the famous physics course of Nobel Prize winner Richard Feynman, that the polarization of light concerns the way in which the electric fieldelectric field which composes it vibrates during the propagation of an electromagnetic wave. A linear polarization describes a field which behaves like an arrow of length and direction oscillating according to a straight line fixed in space but perpendicular to the direction of propagation of the wave. By passing through a material medium or a region where a magnetic field prevails, this state of polarization can change.


Explanations on the polarization of light studied with IXPE. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Marshall Space Flight Center

A magnetic condensation of the atmosphere

In fact, the light produced by the surface of a neutron star, which is solid and consists of a crystal lattice of iron nuclei mostly in a state of extreme density but also of high temperature (on average 100 times that of the surface of the Sun, i.e. about 600,000 K), or polarized by its passage through a thin atmosphere. Its existence and its composition depend in fact on the temperature, the gravity and finally on the magnetic field of the neutron star.

In the case of 4U 0142+61, the researchers found a much lower proportion of polarized light than expected, which is already enough to call into question the existence of an atmosphere. Importantly, the polarization changed drastically at high X-ray energies, following what theoretical models predicted if the neutron star had a purely solid crust surrounded by an outer magnetosphere filled with electric currentselectric currents.

According to Roberto Turolla, from the University of Padua, who led the team of astrophysicists behind these observations: The low-energy polarization tells us that the magnetic field is likely so strong that it caused the atmosphere around the star to change state, turning it into a solid or a liquid, a phenomenon known as condensation. magnetic. Her colleague Silvia Zane specifies that for her: “ It was completely unexpected. I was convinced that there would be an atmosphere. the gasgas of the star has reached a tipping point and has become solid in the same way that water can turn into ice. This is the result of the star’s incredibly strong magnetic field. But, as with water, temperature is also a factor – a hotter gas will require a stronger magnetic field to become solid. A next step will be to observe hotter neutron stars with a similar magnetic field, to study how the interaction between temperature and magnetic field affects the star’s surface properties. »

In fact, the observations concerning the polarization of the X emissions of the magnetar are potentially more talkative still because the researchers deduce from it that they perhaps manifest another possible effect with the very intense magnetic fields of a neutron star. The researchers may thus have also observed the mythical magnetic birefringence of the quantum vacuum.

What is hidden under this esoteric name? Let’s go back to the explanations already given by Futura on this subject.

A nonlinear electrodynamics from the quantum vacuum

This is the equivalent of a phenomenon discovered with light propagating in a material medium immersed in a magnetic field, the Cottom-Moutton effect. It then has two possible speeds of propagation and therefore two indices of refraction, so that there is a splitting of the propagation of light similar to that observed in the famous crystal of Iceland, spar. We talk about birefringence.

But since this birefringence is observed in a vacuum and it is a manifestation of quantum field theory, more precisely of the theory of quantum electrodynamics, the foundations of which were laid in the 1930s by Heisenberg, Pauli, Dirac and Fermi, we therefore speak of magnetic birefringence of the quantum vacuum.

This is a consequence of what is called nonlinear electrodynamics. Normally, light waves pass through each other without affecting each other in a vacuum. But if the electric and magnetic fields of these waves are sufficiently intense, it can be shown, and this is what Werner Heisenberg and his colleague Hans Heinrich Euler did in particular in 1936, that these fields create pairs of particles and d ‘antiparticlesantiparticlesin this case electronselectrons and positrons, from the quantum vacuum. The calculations then show that the Maxwell’s equationsMaxwell’s equations describing the electromagnetic field in a vacuum cease to be linear and that we can, among other things and in a way, cause light rays to collide which will deflect each other like particle collisions. Another aspect of nonlinear electrodynamics is birefringence in vacuum.

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