Improved EM Gauge Condition for GRMHD Simulations with AMR

Monday, November 28, 2011, 10am US central time

The Einstein Toolkit weekly EVO meeting was used for audio and slides, and recordings are available in Theora or compressed, native EVO format. The PDF version of the slides is available, and the talk is based on this paper.

Abstract

We recently developed a new GRMHD code with AMR that evolves the electromagnetic (EM) vector potential A_i instead of the magnetic fields directly. Evolving A_i enables one to use any interpolation scheme on refinement level boundaries and still guarantee that the magnetic field remains divergenceless. As in classical EM, a gauge choice must be made when evolving A_i, and we originally chose a straightforward ``algebraic'' gauge condition to simplify the A_i evolution equation. However, magnetized black hole-neutron star (BHNS) simulations in this gauge exhibit unphysical behavior, including the spurious appearance of strong magnetic fields on refinement level boundaries. This spurious behavior is exacerbated when matter crosses refinement boundaries during tidal disruption of the NS. We demonstrate via an eigenvalue analysis and a numerical study that zero-speed modes in the algebraic gauge, coupled with the frequency filtering that occurs on refinement level boundaries, are responsible for the creation of spurious magnetic fields. We show that the EM Lorenz gauge exhibits no zero-speed modes, and as a consequence, spurious magnetic effects are quickly propagated away, allowing for long-term, stable magnetized BHNS evolutions.

Presenter: Zachariah Etienne

NSF Astronomy & Astrophysics Postdoctoral Fellow, University of Illinois

Numerical Relativity Group Research interests: My research interests span all of computational astrophysics. Much of my past work has focused on simulating the mergers of compact binary systems, including BHNS, BHBH, WDNS, and NSNS. My current work focuses on BHBH mergers, as well as studying MHD effects in BHNS mergers.