CoCoNuT Meeting 2016

We perform viscous general relativistic hydrodynamics simulations for differentially rotating neutron stars which are models of remnants of binary neutron star merger. We first show by axisymmetric longterm simulation that a substantial matter is ejected from such system if viscosity is sufficiently large. Then we show by a three-dimensional simulation that a significant effect of viscous angular momentum transport changes quasi-periodic waveforms of gravitational waves emitted from the merger remnants.

We develop a scheme bridging a gap between numerical models of binary neutron star mergers and long-lived remnant massive neutron stars formed after merger. First, we perform a long-term general relativistic magnetohydrodynamics simulation for about 150ms after merger with a high resolution of 70 m. We find the remnant massive neutron stars is composed of a dense core and envelope which has a large radial extent. Secondly, by evaluating effective alpha viscosity we estimate the angular momentum transport timescale of the remnant massive neutron star core. After the core relaxes into a rigidly rotating star, we model it by a rigidly rotating equilibrium configuration. Thirdly, by assuming angular momentum dissipation mechanism such as magnetic dipole radiation and gravitational radiation, we estimate a lifetime of the remnant massive neutron star core. Finally, we investigate a remnant massive black hole mass, its spin, and accretion torus mass.

I will review recent simulations of binary neutron star mergers performed by the Trento numerical relativity group with the WhiskyMHD code. I will discuss in particular the gravitational wave signals emitted by these objects as well as their connection with the central engine of short gamma-ray bursts. I will describe both models that lead to the formation of a black hole surrounded by an accretion disk and models that produce a long-lived magnetar. I will discuss the effects of magnetic field, equation of state, and mass ratio on the dynamics of these systems.

Binary neutron star mergers are associated with a variety of observable phenomena in the gravitational and electromagnetic spectra. We investigate binary neutron stars in the last milliseconds before and after their merger with full 3D numerical simulations. We explain how we access previously unaccessible regions of the binary neutron star parameter space, e.g. spinning and high mass ratio setups, and we discuss first simulations with generic setups. We also show that recent upgrades allow us to improve the accuracy of the simulation and decrease the phase error of the obtained gravitational waveforms. With this updates our waveforms can be used for validating and improving semi-analytical waveform models.

I will present the status of AdVirgo and the present and future perspectives of GW detection

I will discuss a hierarchical data driven approach to fit results from numerical relativity simulations of non-precessing black hole binaries, and discuss applications to fit the final mass and spin and peak luminosity as functions of the initial black holes, and report on the status of applications to waveform modelling. Special emphasis is given on avoiding overfitting, and on robustness against errors in the numerical relativity data. The procedure may serve in the future as a a simple model to treat the precessing parameter space, or situations with matter, such as binaries containing neutron stars.

Magnetorotational instability (MRI) is thought to be one of the most promising agents amplifying magnetic fields in core-collapse supernovae (CCSNe). However, numerical simulations of MRI in CCSNe are challenging because of very high resolution requirements (so that MRI is not suppressed by numerical dissipation). In my talk I will present results of our recent studies on the influence of numerical schemes and resolution on numerical dissipation. With the help of those results, we can accurately estimate the required resolution for reliable global MRI simulations without the need of convergence tests. I will show how these results can be applied to local and global MRI simulations.

Accretion onto compact objects is the most efficient mechanism to power up a set of astrophysical systems (such as AGN, GRB, X-Ray Binaries, etc. ). Magnetic fields play a major role in enabling the accretion process through the development of magnetic instabilities. Using 3D global GRMHD simulations, we investigated the development of the Papaloizou-Pringle instability (PPI) in 3D magnetized tori accreting onto black holes. The aim of our study is to understand how the PPI interacts with the non-axisymmetric magnetorotational instability (MRI) and to learn whether the disk dynamics is actually dominated by one of these two instabilities. We also studied the effect of a finite resistivity on the accretion flow and the torus dynamics. In particular, we investigated how a finite resistivity might change the relative importance of the two instabilities for the accretion process.

We present the first general relativistic hydrodynamic simulations of individual mini-disks in equal-mass, inspiraling supermassive binary black holes for a suite of initial binary separations. By constructing approximately hydrostationary torii around each black hole, we address open questions regarding the gas dynamics in the central cavity; including mini-disk stability, relativistic tidal truncation, spiral shocks, and disk-disk interactions as the black holes inspiral towards each other. In particular, as the evolution progresses, the tidal effects of the binary companion drive the formation of one- and two-armed spiral density waves. The appearance of one-armed spiral density waves in the mini-disks is not seen in simulations of accretion disks in close binary systems and seems to be related to Post-Newtonian corrections to the tidal perturbation field due to the binary companion.

In neutron stars, the magnetic field evolves mainly through Ohmic dissipation, Hall drift and ambipolar diffusion. The latter process is dominant in the core and is believed to drive the magnetic field evolution in strongly magnetised neutron stars. In this talk, I present the results of our latest work on the core magnetic evolution, which includes for the first time the effects of ambipolar diffusion. Furthermore, I discuss the impact of the ambipolar drift on the magneto-thermal evolution and its importance for magnetars.

A newly born, proto-neutron star or a compact remnant of neutron stars binary merger are expected to rotate differentially and to be important sources of gravitational radiation. A highly accurate, multidomain spectral code is used in order to construct sequences of general relativistic, differentially rotating neutron stars in axisymmetry and stationarity. The high level of accuracy and stability of the code enable us to investigate the solution space corresponding to broad ranges of degree of differential rotation and stellar masses. We find new types of configurations, which were not considered in previous work, mainly due to numerical limitations. The maximum allowed mass for the new types of configurations and moderate degree of differential rotation can be even 2-4 times higher then the maximum mass of non-rotating neutron stars with the same equation of state. Differential rotation can temporarily stabilize a hyper-massive neutron star against gravitational collapse. I will present recent results on differentially rotating neutron stars.

Properties of differentially rotating neutron and quark stars in stationary and axisymmetric configurations have been calculated in the framework of general relativity using a highly accurate multidomain spectral code (cf. Ansorg et al. 2009; Studzińska et al. 2016; Gondek-Rosinska et al. 2016; Szkudlarek et al. 2016). In this talk, we shall give a general overview of results obtained for polytropic equations of state with various adiabatic indices and for the MIT bag model, mainly focussing on the structure of the solution space and on the maximum mass that can be reached (see also companion talk by D. Gondek-Rosinska).

The mutual friction between the neutron superfluid and the charged fluid in the outer core of neutron stars is revisited. We show that the pinning of superfluid vortices to magnetic flux tubes associated with a type II proton superconductor does not only modify the resistivity coefficient but also affects the expression of the mutual friction force. Core-vortex pinning is expected to modify significantly the coupling timescale between the neutron superfluid and the rest of the star, which could have strong implications for pulsar glitches.

We study the effect of superfluidity on torsional oscillations of highly magnetised neutron stars (magnetars) with a microphysical equation of state by means of two-dimensional, magneto-hydrodynamical-elastic simulations. Constant phase, mixed magneto-elastic oscillations might explain the quasi-periodic oscillations observed in magnetar giant flares, since they do not suffer from the additional damping mechanism due to phase mixing, contrary to what happens for continuum oscillations. We provide fits to our numerical simulations that give the oscillation frequencies as functions of magnetic field strength and proton fraction in the core. With these fits we try to constrain some parameters of the neutron star within our model.

Rapidly rotating stars that produce high-mass proto-neutron stars are considered potential progenitors of gamma-ray bursts that might be powered by a collapsar, i.e., a black hole surrounded by an accretion torus, or, if collapse to a black hole does not occur sufficiently early, by a long-lived proto-magnetar. Coupling special relativistic MHD, a pseudo-relativistic gravitational potential, and two-moment neutrino transport, we performed axisymmetric simulations of stars of 35 solar masses that fall into the class of potential GRB progenitors. The results show explosions launched by mechanisms to which neutrino heating, rotation, and magnetic fields contribute to different degrees. The explosions occur in parallel to ongoing accretion onto the PNS, hence allowing for its growth in mass and rotational energy and, as a possible consequence, for later GRBs.

I will present the gravitational wave signal from recent 3D core-collapse simulations carried out by the Garching group. My main focus will be how various hydrodynamical effects leave imprints on the predicted signal and what we can learn about how massive stars explode from observing gravitational waves associated with such an event.

Nuclear shell burning in the final stages of the lives of massive stars is accompanied by strong turbulent convection. The resulting fluctuations aid supernova explosion by amplifying the non-radial flow in the post-shock region. I will talk about our investigation of the physical mechanism behind this amplification using a linear perturbation theory. We model the shock wave as a one-dimensional planar discontinuity and consider its interaction with vorticity and entropy perturbations in the upstream flow. We find that, as the perturbations cross the shock, their total turbulent kinetic energy is amplified by a factor of ~2, while the average linear size of turbulent eddies decreases by about the same factor. These values are not sensitive to the parameters of the upstream turbulence and the nuclear dissociation efficiency at the shock. Finally, I will discuss the implication of our results for the supernova explosion mechanism.

We investigate the interaction of shock waves with acoustic perturbations arising from turbulent convection in core-collapse supernovae using a linear perturbation theory. An interaction of an acoustic wave with a shock generates a downstream field of vorticity and entropy waves. We show that the vorticity waves contain most of the kinetic energy in the post-shock region. We assess the impact of these waves to the explosion condition.

Core-collapse supernovae, the violent deaths of massive stars, are among the most spectacular phenomena in astrophysics: Supernovae can only outshine their host galaxy for weeks; they are laboratories for the behaviour of matter at extreme densities; and they also play a central role for the chemical evolution of galaxies, e.g. as the dominant producers of oxygen and many other elements. Yet the mechanism by which massive stars explode has eluded us for decades. As I shall explain in this talk, this is about to change: Recent first-principle 3D simulations of these events have finally been able to demonstrate that the most popular explosion scenario, the so-called neutrino-driven mechanism, is viable. Including the initial seed asymmetries in the progenitors from convective shell burning could further help to produce even more robust supernova explosion models compatible with observed explosion energies and remnant masses.

High-angular-resolution observations of the immediate vicinity of a black hole will become effective in the near future, thanks to the instruments VLTI/Gravity (2017) and Event Horizon Telescope (2020). This opens the way to tests of general relativity and/or of the black hole nature of the central object. In this respect, I will discuss two alternatives to the Kerr black hole investigated by our group: boson stars and hairy black holes.

Simflowny: automatically generating parallel AMR code for any formulation of the Einstein Equations

In this talk we present a novel second order accurate well balanced Arbitrary-Lagrangian-Eulerian (ALE) finite volume scheme on moving nonconforming meshes for the Euler equations of compressible gasdynamics with gravity in cylindrical coordinates. The main feature of the proposed algorithm is the capability of preserving many of the physical properties of the system: besides being conservative for mass, momentum and total energy, also any known steady equilibrium between pressure gradient, centrifugal force and gravity force can be exactly maintained up to machine precision. Perturbations around such equilibrium solutions are resolved with high accuracy and with minimal dissipation on moving contact discontinuities even for very long computational times. This is achieved by the novel combination of well balanced path-conservative finite volume schemes that are expressly designed to deal with source terms written in terms of nonconservative products with Arbitrary-Lagrangian-Eulerian (ALE) schemes on moving grids, which exhibit only very little numerical dissipation on moving contact waves. In particular, we have formulated a new HLL-type and a novel Osher-type flux that are both able to guarantee the well balancing between the pressure gradient, the centrifugal force and the gravitational force in a gas cloud rotating around a central object. Moreover, to maintain a high level of quality of the moving mesh, we have adopted a nonconforming treatment of the sliding interfaces that appear due to the differential rotation. A large set of different numerical tests has been carried out in order to check the accuracy of the method close and far away from the equilibrium, both, in one and two space dimensions. Finally, a nontrivial test problem in a rotating Keplerian gas disk with variable density shows the greatly reduced dissipation of the new scheme on moving contact discontinuities compared to classical non well balanced Eulerian methods on fixed grids. A significant improvement is obtained both in the radial direction as well as in the angular direction thanks to the novel combination of well-balancing techniques with ALE finite volume schemes on moving nonconforming meshes. The authors of this work are: Elena Gaburro (a), Manuel J. Castro (b), Michael Dumbser (a) ; (a) University of Trento, Italy, (b) University of Málaga, Spain.This work has received funding by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) under the research project STiMulUs, ERC Grant agreement no. 278267.

The evolution of magnetic fields in neutron stars depends strongly on the specific boundary conditions imposed at the stellar surface and the properties of the surrounding magnetosphere. We present various current-free (vacuum) and force-free models of neutron star magnetospheres, describe the resulting boundary conditions, and discuss their implications for the evolution of the neutron star magnetic field as inferred from recent simulations.

From an ongoing examination of magnetic field topologies around rotating black holes an overview of force-free, axisymmetric magnetospheric electrodynamics in Kerr spacetime is presented. Numerical strategies to solve the relativistic Grad-Shafranov equation motivate a closer look on the so called light cylinders from a General Relativity point of view. An outlook on a stability analysis for electromagnetic field configurations employing relativistic magnetohydrodynamics codes (e.g., CoCoNut, Einstein Toolkit) concludes the talk.