The First Open-Source General Relativistic Force-Free Electrodynamics Code

We are writing the first open-source, general relativistic force-free electrodynamics code: GiRaFFE. The goal is to numerically simulate neutron star and black hole magnetospheres. The GiRaFFE is alive! It has passed a number of 1D code validation tests, and 3D tests are underway. We plan to release the code and test suite initial data routines open-source to the community

GiRaFFE: An Open-Source General Relativistic Force-Free Electrodynamics Code

We present GiRaFFE, the first open-source general relativistic force-free electrodynamics (GRFFE) code for dynamical, numerical-relativity generated spacetimes. GiRaFFE adopts the strategy pioneered by McKinney and modified by Paschalidis and Shapiro to convert a GR magnetohydrodynamic (GRMHD) code into a GRFFE code. In short, GiRaFFE exists as a modification of IllinoisGRMHD, a user-friendly, open-source, dynamical-spacetime GRMHD code. Both GiRaFFE and IllinoisGRMHD leverage the Einstein Toolkit's highly-scalable infrastructure to make possible large-scale simulations of magnetized plasmas in strong, dynamical spacetimes on adaptive-mesh refinement (AMR) grids. We demonstrate that GiRaFFE passes a large suite of both flat and curved-spacetime code tests passed by a number of other state-of-the-art GRFFE codes, and is thus ready for production-scale simulations of GRFFE phenomena of key interest to relativistic astrophysics.Comment: 23 pages, 4 figures. Consistent with published versio

Universality and properties of neutron star type I critical collapses

We study the neutron star axisymmetric critical solution previously found in the numerical studies of neutron star mergers. Using neutron star-like initial data and performing similar merger simulations, we demonstrate that the solution is indeed a semi-attractor on the threshold plane separating the basin of a neutron star and the basin of a black hole in the solution space of the Einstein equations. In order to explore the extent of the attraction basin of the neutron star semiattractor, we construct initial data phase spaces for these neutron star-like initial data. From these phase spaces, we also observe several interesting dynamical scenarios where the merged object is supported from prompt collapse. The properties of the critical index of the solution, in particular, its dependence on conserved quantities, are then studied. From the study, it is found that a family of neutron star semi-attractors exist that can be classified by both their rest masses and ADM masses.Comment: 13 pages, 12 figures, 1 new reference adde

Holographic equations of state and astrophysical compact objects

We solve the Tolman-Oppenheimer-Volkoff equation using an equation of state (EoS) calculated in holographic QCD. The aim is to use compact astrophysical objects like neutron stars as an indicator to test holographic equations of state. We first try an EoS from a dense D4/D8/\textoverline {D8} model. In this case, however, we could not find a stable compact star, a star satisfying pressure-zero condition with a radius $R$, $p(R)=0$, within a reasonable value of the radius. This means that the EoS from the D4/D8/\textoverline {D8} model may not support any stable compact stars or may support one whose radius is very large. This might be due to a deficit of attractive force from a scalar field or two-pion exchange in the D4/D8/\textoverline {D8} model. Then, we consider D4/D6 type models with different number of quark flavors, $N_f=1,2,3$. Though the mass and radius of a holographic star is larger than those of normal neutron stars, the D4/D6 type EoS renders a stable compact star.Comment: 12 pages, 9 figure

Properties of Neutron Star Critical Collapses

Critical phenomena in gravitational collapse opened a new mathematical vista into the theory of general relativity and may ultimately entail fundamental physical implication in observations. However, at present, the dynamics of critical phenomena in gravitational collapse scenarios are still largely unknown. My thesis seeks to understand the properties of the threshold in the solution space of the Einstein field equations between the black hole and neutron star phases, understand the properties of the neutron star critical solution and clarify the implication of these results on realistic astrophysical scenarios. We develop a new set of neutron star-like initial data to establish the universality of the neutron star critical solution and analyze the structure of neutron star and neutron star-like critical collapses via the study of the phase spaces. We also study the different time scales involved in the neutron star critical solution and analyze the properties of the critical index via comparisons between neutron star and neutron star-like initial data. Finally, we explore the boundary of the attraction basin of the neutron star critical solution and its transition to a known set of non-critical fixed points

GiRaFFE code [Computer Software]

GiRaFFE leverages the Einstein Toolkit\u27s (ascl:1102.014) highly-scalable infrastructure to create large-scale simulations of magnetized plasmas in strong, dynamical spacetimes on adaptive-mesh refinement (AMR) grids. It is based on IllinoisGRMHD (ascl:2004.003), a user-friendly, open-source, dynamical-spacetime GRMHD code, and is highly scalable, to tens of thousands of cores. Code site: https://bitbucket.org/zach_etienne/wvuthorns/src/master/ Described in: http://adsabs.harvard.edu/abs/2017CQGra..34u5001E Bibcode: 2018ascl.soft10012