Max Planck Institute for Astrophysics | 2020 Mar 13
Magnetars are neutron stars endowed with the strongest magnetic fields observed in the Universe, but their origin remains controversial. ...a team of scientists ... developed a new and unprecedentedly detailed computer model that can explain the genesis of these gigantic fields through the amplification of pre-existing weak fields when rapidly rotating neutron stars are born in collapsing massive stars. The work opens new avenues to understand the most powerful and most luminous explosions of such stars.
Neutron stars are compact objects containing one to two solar masses within a radius of about 12 kilometers. Among them, magnetars are characterized by eruptive emission of X-rays and gamma rays. The energy associated with these bursts of intense radiation is probably related to ultra-strong magnetic fields. Magnetars should thus spin down faster than other neutron stars due to enhanced magnetic braking, and measurements of their rotation period evolution have confirmed this scenario. We thus infer that magnetars have a dipole magnetic field of the order of 1015 Gauss (G), i.e. up to 1000 times stronger than typical neutron stars! While the existence of these tremendous magnetic fields is now well established, their origin remains controversial.
- 3D snapshots of the magnetic field lines in the convective zone inside a newborn neutron star. Inward (outward) flows are represented by the blue (red) surfaces. Left: strong field dynamo discovered for fast rotation periods of a few milliseconds, where the dipole component reaches 1015 G. Right: for slower rotation, the magnetic field is up to ten times weaker. © CEA Sacley
Neutron stars generally form after the collapse of the iron core of a massive star of more than nine solar masses, while the outer layers of the star are expelled into interstellar space in a gigantic explosion called a core-collapse supernova. Some theories therefore assume that neutron star and magnetar magnetic fields could be inherited from their progenitor stars, which means that the fields could be entirely determined by the magnetization of the iron core before collapse. The problem with this hypothesis is, however, that very strong magnetic fields in the stars could decelerate the rotation of the stellar core so that the neutron stars from such magnetized stars would rotate only slowly.
“This would not allow us to explain the huge energies of hypernova explosions and long-duration gamma-ray bursts, where rapidly rotating neutron stars or rapidly spinning black holes are considered as the central sources of the enormous energies,” remarks team member H.-Thomas Janka of MPA. Therefore, an alternative mechanism appears more favorable, in which the extreme magnetic fields could be generated during the formation of the neutron star itself. ...
Magnetar Formation Through a Convective Dynamo in Protoneutron Stars ~ Raphaël Raynaud et al