Periodic Reporting for period 4 - MagBURST (Exploding stars from first principles: MAGnetars as engines of hypernovae and gamma-ray BURSTs)
Reporting period: 2021-11-01 to 2023-04-30
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? The new understanding of magnetic field amplification will be used to improve the realism of these simulations.
During the course of the project, we also proposed a novel scenario for magnetar formation, which is motivated by recent simulations of supernovae explosions suggesting the importance of matter falling back onto the nascent neutron star. Contrary to previous scenarios assuming a fast-rotating progenitor, this novel scenario starts from a slowly rotating progenitor and assumes that the proto-neutron star is spun up by fallback matter. We argued that this can trigger the development of the so-called Tayler-Spruit dynamo while other dynamo processes previously considered are disfavored in this case. Using both semi-analytical modeling and 3D numerical simulations we demonstrated that magnetars can be formed in this scenario with plausible assumptions on the fallback mass. These results are not only important to understand magnetar formation but also for the fields of dynamo theory and stellar evolution as the Tayler-Spruit dynamo had until recently remained elusive in numerical 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. We also showed that the properties of the magnetic field play a major role in determining the gravitational wave signal and the heavy element nucleosynthesis yields. These results demonstrate the need to better describe the origin and properties of the magnetic field to achieve a realistic description of extreme explosions.