Periodic Reporting for period 3 - MagBURST (Exploding stars from first principles: MAGnetars as engines of hypernovae and gamma-ray BURSTs)
Berichtszeitraum: 2020-05-01 bis 2021-10-31
Neutron stars are extremely compact objects containing one to two solar masses within a radius of about 12 kilometers. Among them, magnetars are characterized by eruptive emissions of X-rays and gamma rays. The reservoir of energy associated with these bursts of intense radiation is related to the dissipation of the strongest magnetic fields known in the universe: 10^15 G, i.e. 1000 times stronger than ordinary neutron stars. These objects are therefore unique laboratories to probe physical processes in extreme conditions. While the existence of these extreme magnetic fields is now well established, their origin remains controversial. 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 the interstellar space in a gigantic explosion called a core-collapse supernova. Some theories 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, however, is that very strong magnetic fields in the stars would 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. Therefore, an alternative mechanism appears more favorable, in which the extreme magnetic fields would be generated during the formation of the neutron star itself.
Under the right assumptions (notably sufficiently fast rotation at birth), the magnetar scenario has become a major phenomenological tool to interpret the diversity of observed extreme explosions but its theoretical basis is still insufficiently developed. A major difficulty at present is that the origin of the magnetic field is poorly understood, because the amplification process is sensitive to small length scales which are difficult to resolve in numerical simulations. The goal of this ERC project is to develop an ab initio description of magnetar formation in order to delineate the role they play for the production of gamma-ray bursts and super-luminous supernovae. We plan to address the two following interwoven questions:
1) What is the origin of the gigantic magnetic field observed in magnetars? The physics of the magnetic field amplification in a fast-rotating nascent neutron star will be investigated thoroughly from first principles. By developing the first global protoneutron star simulations of this amplification process, the magnetic field strength and geometry will be determined for varying rotation rates.
2) What variety of explosion paths can be explained by the birth of fast-rotating magnetars? Numerical simulations of the launch of a hypernova explosion and a relativistic GRB jet will provide the first self-consistent description of both events from a millisecond magnetar. The new understanding of magnetic field amplification will be used to improve the realism of these simulations.
In addition to shedding light on Galactic magnetar formation, these results open new avenues to understand the most powerful and most luminous explosions of massive stars. Until now, the main weakness of the millisecond magnetar scenario was to assume an ad hoc strong magnetic field, independent of the fast rotation rate of the neutron star. Our results provide the first numerical demonstration that the right magnetic field can be generated consistently with the fast rotation needed to power the explosion. Another important result has been that the topology of the magnetic field is much more complex than the standard purely dipolar field assumed by most numerical models of magnetorotational explosions. We therefore studied the impact of a more complex geometry of the magnetic field by carrying out numerical simulations of such explosions with different magnetic field configurations. We have shown that the geometry of the magnetic field plays an important role to determine the properties of the explosion. Magnetic fields with more large-scale coherence lead to more energetic explosions that are more collimated in the form of jets. These results demonstrate the need to better describe the origin and properties of the magnetic field to achieve a realistic description of extreme explosions.