rHARM: Accretion and Ejection in Resistive GR-MHD

Turbulent magnetic diffusivity plays an important role for accretion disks and the launching of disk winds. We have implemented magnetic diffusivity, respective resistivity in the general relativistic MHD code HARM. This paper describes the theoretical background of our implementation, its numerical realization, our numerical tests and preliminary applications. The test simulations of the new code rHARM are compared with an analytic solution of the diffusion equation and a classical shock tube problem. We have further investigated the evolution of the magneto-rotational instability (MRI) in tori around black holes for a range of magnetic diffusivities. We find indication for a critical magnetic diffusivity (for our setup) beyond which no MRI develops in the linear regime and for which accretion of torus material to the black hole is delayed. Preliminary simulations of magnetically diffusive thin accretion disks around Schwarzschild black holes that are threaded by a large-scale poloidal magnetic field show the launching of disk winds with mass fluxes of about 50% of the accretion rate. The disk magnetic diffusivity allows for efficient disk accretion that replenishes the mass reservoir of the inner disk area and thus allows for long-term simulations of wind launching for more than 5000 time units.Comment: 21 pages, 43 figures, accepted by Ap

General Relativistic Magnetohydrodynamics Mean-Field Dynamos

Large-scale, ordered magnetic fields in several astrophysical sources are supposed to be originated, and maintained against dissipation, by the combined amplifying action of rotation and small-scale turbulence. For instance, in the solar interior, the so-called α−Ω mean-field dynamo is known to be responsible for the observed 22-years magnetic cycle. Similar mechanisms could operate in more extreme environments, like proto neutron stars and accretion disks around black holes, for which the physical modelling needs to be translated from the regime of magnetohydrodynamics (MHD) and Newtonian gravity to that of a plasma in a general relativistic curved spacetime (GRMHD). Here we review the theory behind the mean field dynamo in GRMHD, the strategies for the implementation of the relevant equations in numerical conservative schemes, and we show the most important applications to the mentioned astrophysical compact objects obtained by our group in Florence. We also present novel results, such as three-dimensional GRMHD simulations of accretion disks with dynamo and the application of our dynamo model to a super massive neutron star, remnant of a binary neutron star merger as obtained from full numerical relativity simulations

Exploiting synergies between neutrino telescopes for the next galactic core-collapse supernova

Observing and characterizing the next galactic core-collapse supernova will be a critical step for neutrino experiments. Extracting information about the supernova progenitors and neutrino properties within minutes after an observation will in particular be crucial in order to optimize analysis strategies at other observatories. Moreover, certain classes of progenitors, with strong magnetic fields, could give rise to gamma-ray bursts but have been underinvestigated to date. In this contribution we propose a strategy to combine results from next-generation neutrino experiments, focusing notably on the determination of the progenitor mass and the neutrino mass ordering. Additionally, we investigate the impact of strong magnetic fields on neutrino observations, and demonstrate the detectability of the associated effects in upcoming experiments

Three-dimensional core-collapse supernovae with complex magnetic structures – I. Explosion dynamics

International audienceMagnetic fields can play a major role in the dynamics of outstanding explosions associated to violent events such as gamma-ray bursts and hypernovae, since they provide a natural mechanism to harness the rotational energy of the central proto-neutron star and power relativistic jets through the stellar progenitor. As the structure of such fields is quite uncertain, most numerical models of MHD-driven core-collapse supernovae consider an aligned dipole as initial magnetic field, while the field’s morphology can actually be much more complex. We present three-dimensional simulations of core-collapse supernovae with more realistic magnetic structures, such as quadrupolar fields and, for the first time, an equatorial dipolar field. Configurations other than an aligned dipole produce weaker explosions and less collimated outflows, but can at the same time be more efficient in extracting the rotational energy from the PNS. This energy is then stored in the surroundings of the PNS, rather than powering the polar jets. A significant axial dipolar component is also produced by models starting with a quadrupolar field, pointing to an effective dynamo mechanism operating in proximity of the PNS surface

Three-dimensional core-collapse supernovae with complex magnetic structures: II. Rotational instabilities and multimessenger signatures

The gravitational collapse of rapidly rotating massive stars can lead to the onset of the low $T/\|W\|$ instability within the central proto-neutron star (PNS), which leaves strong signatures in both the gravitational wave (GW) and neutrino emission. Strong large-scale magnetic fields are usually invoked to explain outstanding stellar explosions of rapidly rotating progenitors, but their impact on the growth of such instability has not yet been cleared. We analyze a series of three-dimensional magnetohydrodynamic models to characterize the effects of different magnetic configurations on the development of the low $T/\|W\|$ and the related multi-messenger features. In the absence of magnetic fields, we observe the growth on dynamical time scales of the low $T/\|W\|$, associated with a strong burst of GW and a correlated modulation of the neutrino emission. However, models with a strong magnetic field show a quenching of the low $T/\|W\|$, due to a flattening of the rotation profile in the first $\sim100$ ms after shock formation caused by the magnetic transport of angular momentum. The associated GW emission is weakened by an order of magnitude, exhibits a broader spectral shape, and has no dominant feature associated with the PNS large-scale oscillation modes. Neutrino luminosities are damped along the equatorial plane due to a more oblate PNS, and the only clear modulation in the signal is due to SASI activity. Finally, magnetized models produce lower luminosities for $\nu_e$ than for $\bar{\nu}_e$, which is connected to a higher concentration of neutron-rich material in the PNS surroundings

A 4th4^{\rm th}-order accurate finite volume method for ideal classical and special relativistic MHD based on pointwise reconstructions

International audienceWe present a novel implementation of a genuinely $4^{\rm th}$-order accurate finite volume scheme for multidimensional classical and special relativistic magnetohydrodynamics (MHD) based on the constrained transport (CT) formalism. The scheme introduces several novel aspects when compared to its predecessors yielding a more efficient computational tool. Among the most relevant ones, our scheme exploits pointwise to pointwise reconstructions (rather than one-dimensional finite volume ones), employs the generic upwind constrained transport averaging and sophisticated limiting strategies that include both a discontinuity detector and an order reduction procedure. Selected numerical benchmarks demonstrate the accuracy and robustness of the method

MRI-driven α−Ω\alpha-\Omega dynamos in protoneutron stars

Magnetars are highly magnetized neutron stars that can produce X-ray and soft gamma-ray emissions and that have a dipole of $10^{14}$ G to $10^{15}$ G. A promising mechanism to explain magnetar formation is magnetic field amplification by the MRI in fast-rotating protoneutron stars (PNS). This scenario is supported by recent global models showing that small-scale turbulence can generate a dipole with magnetar-like intensity. However, the impact of buoyancy and density stratification on the efficiency of the MRI at generating a dipole is still unknown. We assess the impact of the density and entropy profiles on the MRI dynamo in a global model of a fast-rotating PNS, which focuses on its outer stratified region stable to convection. Using the pseudo-spectral code MagIC, we perform three-dimensional Boussinesq and anelastic MHD simulations in spherical geometry with explicit diffusivities. We perform a parameter study in which we investigate the effect of different approximations and of thermal diffusion. We obtain a self-sustained turbulent MRI-driven dynamo, which confirms most of our previous incompressible results once rescaled for density. The MRI also generates a non-dominant equatorial dipole, which represents about 4.3% of the averaged magnetic field strength. Interestingly, in the presence of a density gradient, an axisymmetric magnetic field at large scales oscillates with time, which can be described as a mean-field $\alpha-\Omega$ dynamo. Buoyancy damps turbulence in the equatorial plane but it has overall a relatively weak influence with a realistic high thermal diffusion. Our results support the ability of the MRI to generate magnetar-like large-scale magnetic fields. They furthermore predict the presence of an $\alpha-\Omega$ dynamo in the protoneutron star, which could be important to model in-situ magnetic field amplification in core-collapse supernovae. [abridged