10 results on '"Etienne, Zachariah B"'
Search Results
2. Relativistic magnetohydrodynamics in dynamical spacetimes: Improved electromagnetic gauge condition for adaptive mesh refinement grids.
- Author
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Etienne, Zachariah B., Paschalidis, Vasileios, Yuk Tung Liu, and Shapiro, Stuart L.
- Subjects
- *
RELATIVISTIC astrophysics , *MAGNETOHYDRODYNAMICS , *GAUGE field theory , *NEUTRON stars , *ELECTROMAGNETISM , *MAGNETIC fields , *VECTOR analysis - Abstract
We recently developed a new general relativistic magnetohydrodynamic code with adaptive mesh refinement that evolves the electromagnetic (EM) vector potential A, instead of the magnetic fields directly. Evolving A; 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,, and we chose a straightforward "algebraic" gauge condition to simplify the A, 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 neutron star. Applying Kreiss-Oliger dissipation to the evolution of the magnetic vector potential A, slightly weakens this spurious magnetic effect, but with undesired consequences. 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. Our study demonstrates how the EM gauge degree of freedom can be chosen to one's advantage, and that for magnetized BHNS simulations the Lorenz gauge constitutes a major improvement over the algebraic gauge. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
3. General-relativistic simulations of binary black hole-neutron stars: Precursor electromagnetic signals.
- Author
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Paschalidis, Vasileios, Etienne, Zachariah B., and Shapiro, Stuart L.
- Subjects
- *
RELATIVISTIC cosmology , *BINARY stars , *BINARY black holes , *NEUTRON stars , *SIMULATION methods & models , *MAGNETOSPHERE , *ELECTROMAGNETIC radiation - Abstract
We perform the first general relativistic force-free simulations of neutron star magnetospheres in orbit about spinning and nonspinning black holes. We find promising precursor electromagnetic emission: typical Poynting luminosities at, e.g., an orbital separation of r = 6.6/RNs are Lm ~ 6 X 1042(BNS,p/1013 G)²(MNS/1.4M⊙)² erg/s. The Poynting flux peaks within a broad beam of ~40° in the azimuthal direction and within ~60° from the orbital plane, establishing a possible lighthouse effect. Our calculations, though preliminary, preview more detailed simulations of these systems that we plan to perform in the future. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
4. General-relativistic simulations of black-hole-neutron-star mergers: Effects of tilted magnetic fields.
- Author
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Etienne, Zachariah B., Paschalidis, Vasileios, and Shapiro, Stuart L.
- Subjects
- *
GENERAL relativity (Physics) , *RELATIVISTIC astrophysics , *SIMULATION methods & models , *BLACK holes , *NEUTRON stars , *COSMIC magnetic fields - Abstract
Black-hole-neutron-star (BHNS) binary mergers can form disks in which magnetorotational instability (MRI)-induced turbulence may drive accretion onto the remnant BH, supporting relativistic jets and providing the engine for a short-hard gamma-ray burst (SGRB). Our earlier study of magnetized BHNSs showed that NS tidal disruption winds the magnetic field into a toroidal configuration, with poloidal fields so weak that capturing MRI with full-disk simulations would require ∼108 CPU hours. In that study we imposed equatorial symmetry, suppressing poloidal magnetic fields that might be generated from plasma crossing the orbital plane. Here we show that initial conditions that break this symmetry (i.e., tilted poloidal magnetic fields in the NS) generate much stronger poloidal fields in the disk, indicating that asymmetric initial conditions may be necessary for establishing BHNS mergers as SGRB progenitors via fully general relativistic magnetohydrodynamic simulations. We demonstrate that BHNS mergers may form an SGRB engine under the right conditions by seeding the remnant disk from an unmagnetized BHNS simulation with purely poloidal fields dynamically unimportant initially, but strong enough to resolve MRI. Magnetic turbulence occurs in the disk, driving accretion and supporting Poynting-dominated jet outflows sufficient to power an SGRB. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
5. Importance of cooling in triggering the collapse of hypermassive neutron stars.
- Author
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Paschalidis, Vasileios, Etienne, Zachariah B., and Shapiro, Stuart L.
- Subjects
- *
STELLAR mass , *COOLING , *GRAVITATIONAL collapse , *NEUTRON stars , *GRAVITATIONAL waves , *EQUATIONS of state , *THERMAL analysis , *BLACK holes - Abstract
The inspirai and merger of a binary neutron star (NSNS) can lead to the formation of a hypermassive neutron star (HMNS). As the HMNS loses thermal pressure due to neutrino cooling and/or centrifugal support due to gravitational wave emission, and/or magnetic breaking of differential rotation, it will collapse to a black hole. To assess the importance of shock-induced thermal pressure and cooling, we adopt an idealized equation of state and perform NSNS simulations in full general relativity through late inspiral, merger, and HMNS formation, accounting for cooling. We show that thermal pressure contributes significandy to the support of the HMNS against collapse and that thermal cooling accelerates its "delayed" collapse. Our simulations demonstrate explicitly that cooling can induce the catastrophic collapse of a hot hypermassive neutron star formed following the merger of binary neutron stars. Thus, cooling physics is important to include in NSNS merger calculations to accurately determine the lifetime of the HMNS remnant and to extract information about the neutron star equation of state, cooling mechanisms, bar instabilities and B-fields from the gravitational waves emitted during the transient phase prior to black hole formation. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
6. General relativistic simulations of black-hole-neutron-star mergers: Effects of magnetic fields.
- Author
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Etienne, Zachariah B., Liu, Yuk Tung, Paschalidis, Vasileios, and Shapiro, Stuart L.
- Subjects
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GENERAL relativity (Physics) , *NEUTRON stars , *BLACK holes , *MAGNETIC fields , *RELATIVISTIC mechanics , *SIMULATION methods & models , *ANGULAR momentum (Mechanics) - Abstract
As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at the end of a black-hole-neutron-star (BHNS) binary inspirai, its magnetic fields will be stretched and amplified. If sufficiently strong, these magnetic fields may impact the gravitational waveforms, merger evolution and mass of the remnant disk. Formation of highly-collimated magnetic field lines in the disk + spinning BH remnant may launch relativistic jets, providing the engine for a short-hard GRB. We analyze this scenario through fully general relativistic, magnetohydrodynamic BHNS simulations from inspirai through merger and disk formation. Different initial magnetic field configurations and strengths are chosen for the NS interior for both nonspinning and moderately spinning (aBH/MBH = 0.75) BHs aligned with the orbital angular momentum. Only strong interior (Bmax ∼ 1017 G) initial magnetic fields in the NS significantly influence merger dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning and spinning BH cases, respectively. However, detecting the imprint of even a strong magnetic field may be challenging for Advanced LIGO. Though there is no evidence of mass outflows or magnetic field collimation during the preliminary simulations we have performed, higher resolution, coupled with longer disk evolutions and different initial magnetic field configurations, may be required to definitively assess the possibility of BHNS binaries as short-hard gamma-ray burst progenitors. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
7. HPC-driven computational reproducibility in numerical relativity codes: a use case study with IllinoisGRMHD.
- Author
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Luo, Yufeng, Zhang, Qian, Haas, Roland, Etienne, Zachariah B, and Allen, Gabrielle
- Subjects
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SCIENTIFIC method , *RELATIVISTIC astrophysics , *SCIENTIFIC computing , *NEUTRON stars , *SIMULATION software , *HIGH performance computing - Abstract
Reproducibility of results is a cornerstone of the scientific method. Scientific computing encounters two challenges when aiming for this goal. Firstly, reproducibility should not depend on details of the runtime environment, such as the compiler version or computing environment, so results are verifiable by third-parties. Secondly, different versions of software code executed in the same runtime environment should produce consistent numerical results for physical quantities. In this manuscript, we test the feasibility of reproducing scientific results obtained using the IllinoisGRMHD code that is part of an open-source community software for simulation in relativistic astrophysics, the Einstein Toolkit. We verify that numerical results of simulating a single isolated neutron star with IllinoisGRMHD can be reproduced, and compare them to results reported by the code authors in 2015. We use two different supercomputers: Expanse at SDSC, and Stampede2 at TACC. By compiling the source code archived along with the paper on both Expanse and Stampede2, we find that IllinoisGRMHD reproduces results published in its announcement paper up to errors comparable to round-off level changes in initial data parameters. We also verify that a current version of IllinoisGRMHD reproduces these results once we account for bug fixes which have occurred since the original publication. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
8. General-relativistic simulations of binary black hole-neutron stars: Precursor electromagnetic signals.
- Author
-
Paschalidis, Vasileios, Etienne, Zachariah B., and Shapiro, Stuart L.
- Subjects
- *
RELATIVISTIC astrophysics , *NEUTRON stars - Abstract
We perform the first general relativistic force-free simulations of neutron star magnetospheres in orbit about spinning and nonspinning black holes. We find promising precursor electromagnetic emission: typical Poynting luminosities at, e.g., an orbital separation of r=6.6RNS are LEM~6×1042(BNS,p/1013 G)²(MNS/1.4M⊙)² erg/s. The Poynting flux peaks within a broad beam of ~40° in the azimuthal direction and within ~60° from the orbital plane, establishing a possible lighthouse effect. Our calculations, though preliminary, preview more detailed simulations of these systems that we plan to perform in the future. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
9. General-relativistic simulations of black-hole-neutron-star mergers: Effects of tilted magnetic fields.
- Author
-
Etienne, Zachariah B., Paschalidis, Vasileios, and Shapiro, Stuart L.
- Subjects
- *
GENERAL relativity (Physics) , *NEUTRON stars , *STELLAR mergers - Abstract
Black-hole-neutron-star (BHNS) binary mergers can form disks in which magnetorotational instability (MRI)-induced turbulence may drive accretion onto the remnant BH, supporting relativistic jets and providing the engine for a short-hard gamma-ray burst (SGRB). Our earlier study of magnetized BHNSs showed that NS tidal disruption winds the magnetic field into a toroidal configuration, with poloidal fields so weak that capturing MRI with full-disk simulations would require ~108 CPU hours. In that study we imposed equatorial symmetry, suppressing poloidal magnetic fields that might be generated from plasma crossing the orbital plane. Here we show that initial conditions that break this symmetry (i.e., tilted poloidal magnetic fields in the NS) generate much stronger poloidal fields in the disk, indicating that asymmetric initial conditions may be necessary for establishing BHNS mergers as SGRB progenitors via fully general relativistic magnetohydrodynamic simulations. We demonstrate that BHNS mergers may form an SGRB engine under the right conditions by seeding the remnant disk from an unmagnetized BHNS simulation with purely poloidal fields dynamically unimportant initially, but strong enough to resolve MRI. Magnetic turbulence occurs in the disk, driving accretion and supporting Poynting-dominated jet outflows sufficient to power an SGRB. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
10. HARM3D+NUC: A New Method for Simulating the Post-merger Phase of Binary Neutron Star Mergers with GRMHD, Tabulated EOS, and Neutrino Leakage.
- Author
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Murguia-Berthier, Ariadna, Noble, Scott C., Roberts, Luke F., Ramirez-Ruiz, Enrico, Werneck, Leonardo R., Kolacki, Michael, Etienne, Zachariah B., Avara, Mark, Campanelli, Manuela, Ciolfi, Riccardo, Cipolletta, Federico, Drachler, Brendan, Ennoggi, Lorenzo, Faber, Joshua, Fiacco, Grace, Giacomazzo, Bruno, Gupte, Tanmayee, Ha, Trung, Kelly, Bernard J., and Krolik, Julian H.
- Subjects
- *
STELLAR mergers , *NEUTRON stars , *BINARY stars , *RADIOACTIVE decay , *NEUTRINOS , *NUCLEAR astrophysics , *EQUATIONS of state , *GRAVITATIONAL waves - Abstract
The first binary neutron star merger has already been detected in gravitational waves. The signal was accompanied by an electromagnetic counterpart including a kilonova component powered by the decay of radioactive nuclei, as well as a short Îł -ray burst. In order to understand the radioactively powered signal, it is necessary to simulate the outflows and their nucleosynthesis from the post-merger disk. Simulating the disk and predicting the composition of the outflows requires general relativistic magnetohydrodynamical (GRMHD) simulations that include a realistic, finite-temperature equation of state (EOS) and self-consistently calculating the impact of neutrinos. In this work, we detail the implementation of a finite-temperature EOS and the treatment of neutrinos in the GRMHD code HARM3D+NUC, based on HARM3D. We include formal tests of both the finite-temperature EOS and the neutrino-leakage scheme. We further test the code by showing that, given conditions similar to those of published remnant disks following neutron star mergers, it reproduces both recombination of free nucleons to a neutron-rich composition and excitation of a thermal wind. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
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