170 research outputs found
GRMHD/RMHD Simulations and Stability of Magnetized Spine-Sheath Relativistic Jets
A new general relativistic magnetohydrodynamics (GRMHD) code ``RAISHIN'' used
to simulate jet generation by rotating and non-rotating black holes with a
geometrically thin Keplarian accretion disk finds that the jet develops a
spine-sheath structure in the rotating black hole case. Spine-sheath structure
and strong magnetic fields significantly modify the Kelvin-Helmholtz (KH)
velocity shear driven instability. The RAISHIN code has been used in its
relativistic magnetohydrodynamic (RMHD) configuration to study the effects of
strong magnetic fields and weakly relativistic sheath motion, c/2, on the KH
instability associated with a relativistic, Lorentz factor equal 2.5, jet
spine-sheath interaction. In the simulations sound speeds up to c/1.7 and
Alfven wave speeds up to 0.56 c are considered. Numerical simulation results
are compared to theoretical predictions from a new normal mode analysis of the
RMHD equations. Increased stability of a weakly magnetized system resulting
from c/2 sheath speeds and stabilization of a strongly magnetized system
resulting from c/2 sheath speeds is found.Comment: 5 pages, 5 figures, accepted for publication in Astrophysics and
Space Scienc
A Magnetohydrodynamic Boost for Relativistic Jets
We performed relativistic magnetohydrodynamic simulations of the hydrodynamic
boosting mechanism for relativistic jets explored by Aloy & Rezzolla (2006)
using the RAISHIN code. Simulation results show that the presence of a magnetic
field changes the properties of the shock interface between the tenuous,
overpressured jet () flowing tangentially to a dense external medium.
Magnetic fields can lead to more efficient acceleration of the jet, in
comparison to the pure-hydrodynamic case. A ``poloidal'' magnetic field
(), tangent to the interface and parallel to the jet flow, produces both a
stronger outward moving shock and a stronger inward moving rarefaction wave.
This leads to a large velocity component normal to the interface in addition to
acceleration tangent to the interface, and the jet is thus accelerated to
larger Lorentz factors than those obtained in the pure-hydrodynamic case.
Likewise, a strong ``toroidal'' magnetic field (), tangent to the
interface but perpendicular to the jet flow, also leads to stronger
acceleration tangent to the shock interface relative to the pure-hydrodynamic
case. Overall, the acceleration efficiency in the ``poloidal'' case is less
than that of the ``toroidal'' case but both geometries still result in higher
Lorentz factors than the pure-hydrodynamic case. Thus, the presence and
relative orientation of a magnetic field in relativistic jets can significant
modify the hydrodynamic boost mechanism studied by Aloy & Rezzolla (2006).Comment: 25 pages, 10 figures, accepted for publication in Ap
PIC methods in astrophysics: Simulations of relativistic jets and kinetic physics in astrophysical systems
The Particle-In-Cell (PIC) method has been developed by Oscar Buneman,
Charles Birdsall, Roger W. Hockney, and John Dawson in the 1950s and, with the
advances of computing power, has been further developed for several fields such
as astrophysical, magnetospheric as well as solar plasmas and recently also for
atmospheric and laser-plasma physics. Currently more than 15 semi-public PIC
codes are available which we discuss in this review. Its applications have
grown extensively with increasing computing power available on high performance
computing facilities around the world. These systems allow the study of various
topics of astrophysical plasmas, such as magnetic reconnection, pulsars and
black hole magnetosphere, non-relativistic and relativistic shocks,
relativistic jets, and laser-plasma physics. We review a plethora of
astrophysical phenomena such as relativistic jets, instabilities, magnetic
reconnection, pulsars, as well as PIC simulations of laser-plasma physics
(until 2021) emphasizing the physics involved in the simulations. Finally, we
give an outlook of the future simulations of jets associated to neutron stars,
black holes and their merging and discuss the future of PIC simulations in the
light of petascale and exascale computing.Comment: 117 pages, 44 figures, Invited review article for Living Reviews in
Computational Astrophysics, comments are welcomed, Living Reviews in
Computational Astrophysics, submitted, 2020, the revised version resubmitted
in December 2020, the second revised revision resubmitted in April, 2021,
publishe
Recollimation Shocks in Magnetized Relativistic Jets
We have performed two-dimensional special-relativistic magnetohydrodynamic
simulations of non-equilibrium over-pressured relativistic jets in cylindrical
geometry. Multiple stationary recollimation shock and rarefaction structures
are produced along the jet by the nonlinear interaction of shocks and
rarefaction waves excited at the interface between the jet and the surrounding
ambient medium. Although initially the jet is kinematically dominated, we have
considered axial, toroidal and helical magnetic fields to investigate the
effects of different magnetic-field topologies and strengths on the
recollimation structures. We find that an axial field introduces a larger
effective gas-pressure and leads to stronger recollimation shocks and
rarefactions, resulting in larger flow variations. The jet boost grows
quadratically with the initial magnetic field. On the other hand, a toroidal
field leads to weaker recollimation shocks and rarefactions, modifying
significantly the jet structure after the first recollimation rarefaction and
shock. The jet boost decreases systematically. For a helical field, instead,
the behaviour depends on the magnetic pitch, with a phenomenology that ranges
between the one seen for axial and toroidal magnetic fields, respectively. In
general, however, a helical magnetic field yields a more complex shock and
rarefaction substructure close to the inlet that significantly modifies the jet
structure. The differences in shock structure resulting from different field
configurations and strengths may have observable consequences for disturbances
propagating through a stationary recollimation shock.Comment: 14 pages, 15 figures and 1 table, accepted for publication in Ap
Reductive Extraction of Actinides and Lanthanides from Molten Chloride Salt into Liquid Zinc
As one of the basic investigations on the group partitioning of actinides and lanthanides by pyrochemical reductive extraction system, their distribution behavior in a binary phase system of molten chloride and liquid zinc was studied, Generally speaking, actinides were a litte more easily reduced and extracted from the salt phase into the metal phase than lanthanides. However, the separation factors which were the differences in the distribution coefficient between actinides and lanthanides were not so large even at lower temperatures, and the group partitioning of these elements seemed less attractive in this system. The present results were much the same as those in the LiF-BeF_2/Zn system and the effect of the selection of the salt phase on the separation factors were hardly observed. For some details, the thermodynamic quantities of actinides and lanthanides in the system were calculated from the equilibrium distributions and discussed
New herpetofaunal records from Gunung Mulu National Park and its surrounding areas in Borneo
Gunung Mulu National Park (GMNP) in northwestern Borneo is marked by high species diversity and diverse environments. We present one new amphibian and ten new reptile records from GMNP and its surrounding area. In the records, Asthenodipsas jamilinaisi and Garthius chaseni were newly recorded in the Sarawak State. We also present the first record of Cyrtodactylus muluensis from outside of GMNP and the second record of Opisthotropis typica from the park. Combined with previous information, a total of 108 amphibians and 104 reptiles are known from GMNP, and their preferred habitat types are diverse. Furthermore, observed male-male combat of Dopasia buettikoferi is the first detailed description of the genus. Two color morphs of D. buettikoferi had an identical ND2 haplotype and appeared to be the same species. The present study provides new information about Bornean amphibians and reptiles, and also emphasizes the importance of continuous monitoring
General Relativistic Radiative Transfer: Applications to Black-Hole Systems
We present general relativistic radiation transfer formulations which include opacity effects due to absorption, emission and scattering explicitly. We consider a moment expansions for the transfer in the presence of scattering. The formulation is applied to calculation emissions from accretion and outflows in black-hole systems. Cases with thin accretion disks and accretion tori are considered. Effects, such as emission anisotropy, non-stationary flows and geometrical self-occultation are investigated. Polarisation transfer in curved space-time is discussed qualitatively
Magnetohydrodynamic Effects in Propagating Relativistic Jets: Reverse Shock and Magnetic Acceleration
We solve the Riemann problem for the deceleration of an arbitrarily
magnetized relativistic flow injected into a static unmagnetized medium in one
dimension. We find that for the same initial Lorentz factor, the reverse shock
becomes progressively weaker with increasing magnetization \sigma (the
Poynting-to kinetic energy flux ratio), and the shock becomes a rarefaction
wave when \sigma exceeds a critical value, \sigma_c, defined by the balance
between the magnetic pressure in the flow and the thermal pressure in the
forward shock. In the rarefaction wave regime, we find that the rarefied region
is accelerated to a Lorentz factor that is significantly larger than the
initial value. This acceleration mechanism is due to the strong magnetic
pressure in the flow. We discuss the implications of these results for models
of gamma-ray bursts and active galactic nuclei.Comment: 13 pages, 3 figures, Accepted to publication in ApJ Letter
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