An international research team, including the University of Geneva (UNIGE), has for the first time used numerical simulations to recreate the formation and evolution of a magnetar. This significant improvement in the comprehension of these stars was published in the journal Nature Astronomy.
A simulated magnetar with magnetic field lines and surface temperature (temperature increases with color, tending from red to yellow). Image Credit: Raphaël Raynaud (LMPA/AIM/IRFU/DRF/CEA Saclay)
Magnetars are neutron stars that possess the strongest magnetic fields in the Universe. These enormously dense objects are at the heart of extreme events like hypernovae, rapid radio bursts, and gamma-ray bursts. However, their origin remains unknown.
At the end of their lives, stars with eight times the mass of the Sun experience core collapse owing to gravity. This moment signals the start of the star's explosion into a supernova: the outer layers are ejected, while the core shrinks fiercely, becoming a neutron star, the densest known object in the universe. One teaspoon of neutron star matter weighs a billion tons.
While neutron stars are normally seen in radio waves, some produce intense X-ray and gamma ray bursts. They are widely known as “magnetars”' because their emissions are assumed to be created by the dissipation of enormous magnetic fields that are a million billion times more intense than those on Earth.
The Mystery of Magnetar Origin
Scientists are working to uncover the origin of magnetar magnetic fields, which play a crucial role in the luminous phenomena associated with them. While several theories have been proposed, the leading explanation suggests that these magnetic fields form through dynamo action in the proto-neutron star just seconds after the explosion begins.
Dynamo action enables a conducting fluid, such as plasma, with sufficiently complex motions, to amplify and maintain its own magnetic fields against the diffusive effects, which weaken them. This amplification effect is undoubtedly at the origin of the majority of astrophysical magnetic fields, such as those of the Sun or Earth. Unlike the others, this theory is supported by a large number of numerical simulations.
Paul Barrère, Study Second Author and Postdoctoral Researcher, Department of Astronomy, University of Geneva
A New Magnetar Formation Scenario
Many of these dynamos require the core of the original star to spin very quickly to function properly. However, these rotational velocities are poorly known due to the scarcity of observations. Researchers Paul Barrère, Jérôme Guilet, and Raphaël Raynaud from CEA Saclay's Department of Astrophysics investigated an alternative scenario, which suggests that the proto-neutron star is spun up by some of the matter ejected during the supernova and later falling back onto the star's surface.
Barrère added, “This renders our new formation scenario independent of the progenitor star rotation.”
The Tayler-Spruit dynamo is the preferred mechanism for amplifying the magnetic field in this proto-neutron star.
“This mechanism feeds off the difference of rotation inside the star and an instability of the magnetic field. This dynamo is well known to researchers working on stars, as it could explain core rotation in stars,’’ explained the researcher.
Simulating Magnetar Evolution
Despite its importance, this new scenario focuses only on the initial few seconds following the supernova, which is quite brief in comparison to the age of the observed magnetars. Collaboration with scientists from the Universities of Newcastle and Leeds, experts in neutron star evolution, was therefore essential in developing the first numerical simulation of a neutron star's evolution over a million-year timescale. This simulation modeled the effects of an initial complex magnetic field generated by the Tayler-Spruit dynamo
“The combination of our expertise has, for the first time, bridged the gap between our studies of formation in proto-neutron stars and research on the evolution of evolved neutron stars,’’ stated Paul Barrère.
The neutron star generated in this study has the same observable features as the so-called weak-field magnetars found in 2010. These magnetars' magnetic dipoles are ten to one hundred times weaker than those of traditional magnetars. This study concludes that these magnetars are most likely produced in neutron protostars accelerated by supernova matter accretion and powered by the Tayler-Spruit dynamo.
“Our work represents a major breakthrough in our understanding of magnetars and opens very interesting new perspectives in the study of other dynamo effects. Our results suggest that each dynamo leaves its imprint on the complex magnetic field configuration and therefore on the observed emission from magnetars. While the Tayler-Spruit dynamo is associated with low-field magnetars, we hope to identify in the future the mechanisms associated with the other magnetars,’’ concluded Barrère.
Journal Reference:
Igoshev, A. et. al. (2025) IA connection between proto-neutron-star Tayler–Spruit dynamos and low-field magnetars. Nature Astronomy. doi.org/10.1038/s41550-025-02477-y