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Exploding stars from first principles: MAGnetars as engines of hypernovae and gamma-ray BURSTs

Periodic Reporting for period 2 - MagBURST (Exploding stars from first principles: MAGnetars as engines of hypernovae and gamma-ray BURSTs)

Reporting period: 2018-11-01 to 2020-04-30

This project aims to provide a theoretical understanding of the most extreme stellar explosions. For instance, superluminous supernovae emit a hundred times more light than usual supernovae, while others, called hypernovae, are characterized by a kinetic energy larger by a factor of ten and sometimes associated with a gamma-ray burst lasting several tens of seconds. These events are among the most luminous events observed up to very large distances and are therefore useful to probe the early universe. They are also by themselves important events in astrophysics because they have a decisive impact on their environment, in particular through their feedback on stellar formation and for the nucleosynthesis of heavy elements. Our knowledge of these events and their diversity has improved significantly in the past five years and should continue to progress in the next decade: synoptic surveys from the LSST telescope (Large Synoptic Survey Telescope, under construction) will detect up to a hundred times more supernovae than at present, and the mission space SVOM will be able to provide the localization and spectral characteristics of a multitude of gamma-ray bursts with unmatched precision and responsiveness. The rapid evolution of our knowledge of stellar explosions makes it all the more necessary to develop theoretical models capable of accounting for it. A promising scenario to explain these extreme explosions is the formation of a magnetar: a class of neutron star whose magnetic field is extraordinarily strong.

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.
To explain the genesis of magnetars’ magnetic field, we developed new and unprecedently detailed numerical models describing the amplification of pre-existing weak fields when neutron stars are born with fast rotation. The two most likely mechanisms consist either in a convective dynamo similar to planetary and stellar dynamos or in tapping into the differential rotation energy thanks to the magnetorotational instability (MRI). Although they are well studied in other astrophysical contexts, the effectiveness of these two physical mechanisms under the physical conditions specific to protoneutron stars was poorly understood. Their capacity to generate a magnetic field coherent over the whole neutron star was particularly uncertain, although this is crucial to explain the properties of galactic magnetars and to power an explosion. We produced the first 3D numerical simulations of each of these two processes in a protoneutron model describing its full angular extent. To develop these models, we have adapted numerical methods from the field of stellar and planetary dynamos to the context of a proto-neutron star with ultra-dense and hot matter. These simulations showed for the first time the generation of a magnetic field consistent with the observational constraints from galactic magnetars. They showed in particular the appearance for fast enough rotations of a new branch of convective dynamo with much larger magnetic fields. Not only does the dipolar magnetic field reach 10^15 G, but the toroidal component is even stronger by a factor ten.

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.
The results obtained in the first half of the ERC period have already led to an important step forward in the theory of magnetar formation by developing the first numerical simulations describing the generation of a magnetar-strength dipolar magnetic field. For the tractability of this first step, we have made several simplifying assumptions which can now be improved upon in order to reach a better predictive power. First of all, it will now be important to use a more realistic description of the protoneutron star structure and of its time evolution. The simulations of the magnetorotational instability (MRI) indeed used an idealized protoneutron star model, while convective dynamo simulations were restricted to the convectively unstable region deep inside the star. We plan to develop a model describing realistically the full protoneutron star structure. This will notably allow us to study the interaction between the convective dynamo and the MRI that develops in the region lying between the convective zone and the surface. This is particularly important for the launch of the explosion which starts from the protoneutron star surface. Another important restriction of the current simulations is their use of moderate values of magnetic Prandtl number (Pm), while protoneutron stars lie in the regime of very large Pm. We expect that local simulations of the MRI will bring a novel understanding of dynamo action in this regime. Finally, concerning numerical simulations of magnetar-driven explosions, we plan to improve their realism by developing new strategies to include the physics of magnetic field amplification taking place at unresolved subgrid scales.
Magnetic field lines in a protoneutron star convective dynamo (Raynaud et al 2020).