Moving ‘sunspots’ seen on neutron star’s crust

Neutron stars are celestial bodies exotic whose existence was predicted during the 1930s. They can form when a star exceeds 8 masses solar. There then comes a time when the thermonuclear reactions producing the pressure of radiation opposing thecollapse under their own gravity of these stars cease. Their heart of iron will implode producing a series of complex phenomena discussed in the CEA video at the bottom of this article and which leads to an explosion, more precisely what is called a supernova type II. In many cases, all that remains is a compact star bringing together in a volume of a few tens of kilometers in diameter approximately one solar mass (in the others, it is a black hole which is formed). The contraction forces a majority of the electrons and protons of the ferrous core of the star before the explosion to combine following a disintegration reaction beta inverse, which produces neutrons, hence the name of these exotic stars.

The first works, laying the foundation on which the theories of neutron stars and that of gravitational collapse leading to the formation of a black hole will be built in the late 1950s and early 1960s, date back to 1939 and we owe them to Robert Oppenheimer. These are articles written in collaboration with his students at the time: On Massive Neutron Cores », with Georges Volkoff, and « On Continued Gravitational Contraction with Hartland Snyder.

From pulsars to magnetars

It will still be necessary to wait for 1967, with the observation of a first pulsar by Jocelyn Bell then its interpretation asneutron star in rotation by Thomas Gold and Franco Pacini, so that the community of astrophysicists is convinced of the existence of these stars fascinating with strange properties. They are made up of a matter so dense that a teaspoon would weigh as much as a mountain on Earth. The field of gravitation y is therefore so intense that it is necessary to appeal to the theory of general relativity to describe them and they are even laboratories for testing other relativistic theories of gravitation such as, for example, that of entangled relativity.

It has been discovered for some years now that some neutron stars have extraordinarily strong magnetic fields. We are therefore talking about magnetar to designate these magnetic stars. An ordinary neutron star is already strongly magnetized because the intensity of its magnetic field is on average up to 10 billion times more intense than that of a magnet of fridge. But a magnetar usually has one a thousand times stronger.

Magnetars are rotating like the Sun and, like all neutron stars, they possess a high surface temperature. It can therefore produce phenomena similar to those existing on the surface of the Sun with its magnetic plasma, that is to say in particular coronal loops.

Neutron stars with coronal loops?

Remember that the coronal loops, like the sunspotsare strongly linked to the solar magnetic field and constitute active regions of the Sun where the phenomena eruptive occur most frequently with lines of intense and dynamic magnetic fields that can store a lot ofenergy before releasing her.

However, the surface of a neutron star is believed to be largely solid and made up of iron cores while its interior is more and more dominated by neutrons packed on top of each other as one goes deeper into the star, until the pressures and temperatures lead to the appearance of a state of matter nuclear power still poorly understood and probably with the formation of a quagma.

Still, we can try to better understand the structure and evolution of neutron stars in many ways, in particular by studying their X-ray emissionsand this is what astrophysicists do with the instrument Neutron star Interior Composition Explorer (nicer) of NASA on board theISS.

Nicer allowed them to take a closer look at a previously spotted magnetar named SGR 1830-0645 (SGR 1830 for short). It was particularly marked by a strong eruption in X-rays on October 10, 2020, spotted from space by the Neil Gehrels Observatory Swift from NASA.

Plate tectonics on a neutron star?

SGR 1830 is located somewhere in the Milky Way towards the constellation Sobieski’s Shield (Scutum), a small constellation that lies just east of the Serpent’s Tail. As explained in the video above and a post in The Astrophysical Journal Letters, Swift’s gaze, as well as that of Nicer, shows a series of periodic pulses in the X-ray domain implying that the detected neutron star rotates on its axis every 10.4 seconds or so.

However, always on closer inspection, the impulses were revealed to be triple and finally double. The best way to interpret these observations is to initially involve three regions that are particularly hot and therefore bright because at temperatures exceeding one million degrees. In fact, the theory about these regions is that they are the equivalent of sunspots and the feet of the coronal loops of the Sun. Two of them would have moved and eventually merged, reducing these particularly bright regions from three to two. During these trips the crust of the star would have melted locally and movements similar to those of the tectonic plates on Earth would also have occurred in interaction with the dynamics of magnetic coronal loops.

What is certain is that for the first time we see the evolution of sorts of “sunspots” on the surface of a neutron star.