An explanation for low-field magnetars 🧲

Magnetars are neutron stars characterized by some of the most powerful magnetic fields in the Universe, as well as their intense X-ray and gamma-ray emissions and outbursts.

An international team of scientists, including researchers from the Astrophysics Department of CEA-Paris Saclay, has modeled the formation of their magnetic field, induced by the Tayler-Spruit dynamo, and its evolution over periods of several hundred thousand years.


This dynamo is generated by the fall of matter onto the neutron star just after its formation, following the explosion of the parent star into a supernova. The simulation results are consistent with observations of low-B magnetars, neutron stars with dipolar magnetic fields 10 to 100 times weaker than those of classical magnetars.

These results represent a major breakthrough by solving a scientific mystery that has persisted since the discovery of these magnetars in 2010. They also indicate that low-field magnetars form through a different process than classical magnetars, likely due to variations in neutron star dynamos during their formation.

The study was published in the journal Nature Astronomy.

The Tayler-Spruit dynamo mechanism...

Neutron stars are the remnants of massive stars that have exhausted their fuel and then exploded as supernovae, ejecting most of their outer layers. Among them, magnetars stand out due to their intense magnetic fields, which can reach 10¹⁵ Gauss, about ten billion times stronger than the fields generated by humans. These extreme fields make magnetars bright transient sources of X-rays and gamma rays. A key question remains: understanding the precise origin of these fields and their evolution over millions of years.


In 2022, a team from the Astrophysics Department (DAp) of CEA Saclay proposed an innovative scenario to explain the formation of these extreme magnetic fields, based on the Tayler-Spruit dynamo, a process that converts plasma motion into a magnetic field. The Tayler-Spruit dynamo is particularly activated when matter ejected during the supernova explosion falls back onto the young neutron star.

In 2023, a detailed study using three-dimensional numerical simulations succeeded in reproducing the observed magnetic field strengths of magnetars (cf. Figure 1). However, these simulations were limited to the first ten seconds following the formation of the proto-neutron star. It remained to understand how these fields evolve over long periods.

... could give rise to low-field magnetars

To study the long-term evolution of magnetars whose magnetic field is generated by the Tayler-Spruit dynamo, the team from the Astrophysics Department (DAp) of CEA Saclay collaborated with researchers from the universities of Newcastle and Leeds, specialists in the evolution of neutron stars over timescales reaching several hundred thousand years.

Dr. Andrei Igoshev, lead author of the study and researcher at the School of Mathematics, Statistics, and Physics at Newcastle University, explains: "This study shows that this process plays a crucial role in the formation of low-field magnetars' magnetic fields through the Tayler-Spruit dynamo."


The numerical simulations conducted in this study (cf. Figure 2) reproduce the main observed characteristics of low-field magnetars ("low-B"), including:

- Weak dipolar magnetic fields: These magnetars exhibit magnetic fields 10 to 100 times weaker than those of classical magnetars, with values below 10¹³ Gauss.

- X-ray light curves: Due to their extremely high temperatures, magnetars primarily emit in the X-ray range. The simulations accurately reproduce this thermal emission modulated by hot spots on the star's surface.

- X-ray bursts and flares: The powerful magnetic field deforms the neutron star's crust, sporadically causing small ruptures. These deformations trigger bright flashes in X-rays and gamma rays. The simulations show that the conditions necessary for these events are well reproduced.

- Slow rotation periods: Like classical magnetars, low-field magnetars exhibit long rotation periods compared to other neutron stars. One hypothesis is that these magnetars could be aged versions of classical magnetars, whose initially strong dipolar magnetic field would have extracted their angular momentum before gradually dissipating. This study proposes another explanation: their slow rotation period could result from interaction with surrounding matter from the supernova fallback, which would slow the star's rotation after generating the magnetic field.

A scientific mystery solved!

Through this study, the researchers have shown that the Tayler-Spruit dynamo, activated by the fallback of matter from the supernova explosion, is a key mechanism behind the formation of low-field magnetars. This work demonstrates that these objects are not necessarily evolved remnants of classical magnetars but can result from a dynamic process from their birth. This scenario thus provides a complete and coherent explanation of the observed phenomena, while solving a scientific mystery that has persisted since their identification in 2010.

"By directly connecting the dynamo process to the observable properties of magnetars, these simulations, the first to describe the long-term evolution of the magnetic field from a coherently generated field by such a process, open a new path to constrain the formation of these mysterious objects," enthuses Jérôme Guilet, a researcher at DAp and one of the study's authors.