Session J8: Simulations of General Relativistic Astrophysical Phenomena

On the detectability of dual jets from binary black holes

We revisit the suggestion that dual jets can be produced during the inspiral and merger of supermassive black holes when these are immersed in a force-free plasma threaded by a uniform magnetic field. By performing independent calculations and by computing the electromagnetic emission in a way which is consistent with estimates using the Poynting flux, we show that a dual-jet structure is present in our simulations, but energetically subdominant with respect to a non-collimated and predominantly quadrupolar emission, which is similar to the one computed when the binary is in electrovacuum. While our findings set restrictions on the detectability of dual jets from coalescing binaries, they also increase the chances of detecting an EM counterpart from these systems.   

Modeling charged particles around dual jets from coalescing binary black holes

Electromagnetic emission produced during the in-spiral and merger of supermassive black holes could provide, together with gravitational waves, the smoking gun to identify these astrophysical events. To gain further insight on the mechanisms responsible for the electromagnetic emission, we present results from simulations of pressure-less charged matter in the vicinity of merging black holes immersed in a magnetic field. In particular, we discuss the effect that the dual jets from coalescing binary black holes have on the dynamics of the charged particles.   

Tidal Disruption of a Main Sequence Star by a Super-massive Black Hole Binary

We present results from a numerical relativity study of the tidal disruption of a main sequence star by a super-massive black hole binary during coalescence. We compare the disruption process with the traditional case of a single massive black hole. In particular, we focus on the geometry of the ejecta and the fall back rate of the debris. We also elaborate on the observational signatures and the possibility of detecting the tidal flares from these disruptive encounters.   

Three-dimensional Simulations of Stellar Core Collapse using Multiblocks

Three dimensional simulations without symmetry assumptions are crucial for the proper modeling of stellar core collapse and collapsar formation. These simulations allow to study the 3D dynamics of convection, standing accretion-shock instability, protoneutron star pulsation, rotational effects and hydrodynamic turbulence. Unfortunately, full 3D simulations based on Cartesian meshes are very expensive, even with adaptive mesh refinement. A much more efficient way of discretizing the computational domain is given by multiblock schemes. Multiblock schemes allow to use grids adapted to the topology of the problem. In the context of stellar core collapse, the problem topology is spherical. Hence the application of spherical grids with a fixed angular resolution will lead to a tremendeous performance benefit compared to Cartesian meshes. I present a new general relativistic hydrodynamic multiblock code based on cell-centered adaptive mesh refinement. I apply the scheme to full three dimensional simulations of rotating stellar core collapse.   

The relative stability of black hole threshold solutions in gravitational collapse

We present numerical studies of the relative stability of critical solutions in problems of gravitational collapse. These strong-field solutions to Einstein equations, initially discovered by M. Choptuik, arise at the threshold of black hole formation. We study the evolution of systems with multiple matter sources (such as scalar or Yang-Mills fields) that exhibit the same type of threshold solution when studied individually and only interact with each other gravitationally. Given the unstable nature of critical solutions, the central question that we address is how does matter of one type behave in the presence of a critical solution of another type of matter. Preliminary results, using adaptive grid techniques to solve Einstein equations coupled to matter, indicate that the near-critical solution of the combined system seems to switch from one type of threshold to another, as the critical point is approached in parameter space. The overall dynamics (exhibiting time-periodicity or self-similarity) depends on the relative amounts of energy present in the system and on the overlapping region of the fields.   

Black hole free energy during charged collapse

Numerical work on gravitational collapse in isotropic coordinates has recently shown that the negative of the gravitational Lagrangian approaches the (Helmholtz) free energy of a Schwarzschild black hole at late times of the collapse. We investigate numerically this association during the collapse of a charged scalar field to a Reissner-Nordstr\"om (RN) black hole in isotropic coordinates. Charged collapse yields a large outgoing matter wave in the exterior region but this has a negligible effect on the interior. The thermodynamics via the free energy can therefore be investigated by focusing on the interior. We find that the percentage discrepancy between the numerical value for the Lagrangian and the analytical expression for the free energy reach values as low as $3\%$ depending on the initial state. As a consistency check, we also implement a procedure for prolonging the evolution of the exterior region. The matter Lagrangian approaches zero everywhere (interior and exterior) showing clearly that the entropy of the charged black hole is gravitational in origin.