Neutron stars are extraordinarily compact objects produced by the core collapse of massive stars that have exhausted their fuel. Some of them, known as magnetars, have colossal magnetic fields that are nearly one thousand times stronger than those of neutron stars and emit bursts of X-rays and gamma rays. Our own galaxy is home to about thirty of them. So why do some stars become magnetars and not conventional neutron stars?

Researchers have succeeded in simulating the changes in the magnetic field during the first few seconds after the formation of a magnetar, using magneto-hydrodynamic models derived from models describing the magnetism of the Sun or the Earth. As the iron core of the star collapses on itself, it cools down by emitting a large amount of neutrinos, creating convective motions capable of amplifying the magnetic field by "dynamo action". If the rotation of the star on itself is fast enough to alter the force balance governing the intensity of the magnetic field, it can reach spectacular levels that are the signature of magnetars. A high rotational speed therefore seems to be an essential ingredient in the formation of a magnetar.

In contrast to alternative theories, this approach offers a new perspective on massive star explosions involving phenomenal energies, such as "superluminous" supernovae or "hypernovae". In summary, it makes the connection between the extreme magnetic field and the rotation speeds necessary to explain the driving force behind the strongest explosions known to date.

The calculations for this study were carried out on the Occigen supercomputer at the Centre informatique national de l'enseignement supérieur (Cines).