469 research outputs found
Transformation of the Poynting flux into the kinetic energy in relativistic jets
The acceleration of relativistic jets from the Poynting to the matter
dominated stage is considered. The are generally two collimation regimes, which
we call equilibrium and non-equilibrium, correspondingly. In the first regime,
the jet is efficiently accelerated till the equipartition between the kinetic
and electro-magnetic energy. We show that after the equilibrium jet ceases to
be Poynting dominated, the ratio of the electro-magnetic to the kinetic energy
decreases only logarithmically so that such jets become truly matter dominated
only at extremely large distances. Non-equilibrium jets remain generally
Poynting dominated till the logarithmically large distances. In the only case
when a non-equilibrium jet is accelerated till the equipartition level, we
found that the flow is not continued to the infinity but is focused towards the
axis at a finite distance from the origin.Comment: Submitted to MNRAS Minor changes in the Conclusion
Stellar explosions powered by the Blandford-Znajek mechanism
In this letter we briefly describe the first results of our numerical study
on the possibility of magnetic origin of relativistic jets of long duration
gamma ray bursters within the collapsar scenario. We track the collapse of
massive rotating stars onto a rotating central black hole using axisymmetric
general relativistic magnetohydrodynamic code that utilizes a realistic
equation of state of stellar matter, takes into account the cooling associated
with emission of neutrinos, and the energy losses due to dissociation of
nuclei. The neutrino heating is not included. We describe the solution for one
particular model where the progenitor star has magnetic field G. The
solution exhibits strong explosion driven by the Poynting-dominated jets whose
power exceeds . The jets originate mainly from the
black hole and they are powered via the Blandford-Znajek mechanism. The full
details of the simulations together with the results of parameter study will be
presented elsewhere. A number of simulation movies can be downloaded from
http://www.maths.leeds.ac.uk/~serguei/research/movies/anim.htmlComment: minor revision, accepted by MNRAS Letters, simulation movies can be
downloaded from
http://www.maths.leeds.ac.uk/~serguei/research/movies/anim.htm
Polarization in the inner region of Pulsar Wind Nebulae
We present here the first effort to compute synthetic synchrotron
polarization maps of Pulsar Wind Nebulae (PWNe). Our goal is to highlight how
polarization can be used as an additional diagnostic tool for the flow
structure in the inner regions of these nebulae. Recent numerical simulations
suggest the presence of flow velocities ~0.5 c in the surroundings of the
termination shock, where most of the high energy emission comes from. We
construct polarization maps taking into account relativistic effects like
Doppler boosting and position angle swing. The effect of different bulk
velocities is clarified with the help of a toy-model consisting of a uniformly
emitting torus. We also present a map based on recent numerical simulations of
the entire nebula and compare it with presently available data. The comparison
with upcoming high resolution observations could provide new insight into the
inner structure of the nebula and put constraints on the geometrical properties
of the magnetic field.Comment: Accepted for publication on A&A, 6 pages, 2 figure
Simulated synchrotron emission from Pulsar Wind Nebulae
A complete set of diagnostic tools aimed at producing synthetic synchrotron
emissivity, polarization, and spectral index maps from relativistic MHD
simulations is presented. As a first application we consider here the case of
the emission from Pulsar Wind Nebulae (PWNe). The proposed method is based on
the addition, on top of the basic set of MHD equations, of an extra equation
describing the evolution of the maximum energy of the emitting particles. This
equation takes into account adiabatic and synchrotron losses along streamlines
for the distribution of emitting particles and its formulation is such that it
is easily implemented in any numerical scheme for relativistic MHD. Application
to the axisymmetric simulations of PWNe, analogous to those described by Del
Zanna et al. (2004, A&A, 421, 1063), allows direct comparison between the
numerical results and observations of the inner structure of the Crab Nebula,
and similar objects, in the optical and X-ray bands. We are able to match most
of the observed features typical of PWNe, like the equatorial torus and the
polar jets, with velocities in the correct range, as well as finer emission
details, like arcs, rings and the bright knot, that turn out to arise mainly
from Doppler boosting effects. Spectral properties appear to be well reproduced
too: detailed spectral index maps are produced for the first time and show
softening towards the PWN outer borders, whereas spectral breaks appear in
integrated spectra. The emission details are found to strongly depend on both
the average wind magnetization (here approximately 2%), and on the magnetic
field shape.Comment: 14 pages, submitted to A&
Large electric field effects on the resistance of LaCaMnO microstructures
We investigate electric field effects in thin film microbridges of
LaCaMnO with the focus on the regime of metal-insulator
transition. A mechanically milled SrTiO substrate is used as a backgate
dielectric. Inside the metal-insulator transition we find a strong unipolar
field-induced reduction in resistance, as well as a suppression of the
nonlinear features in the I-V curves we observed earlier. We associate the
observed effects with a phase separated state in which metallic regions coexist
with short range correlated polaron regions. When the glassy polaron phase has
fully developed, and closes off the microbridge, the field effects disappear
leaving the strongly nonlinear behavior of the transport current unaltered.Comment: 3 pages, 5 figure
Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields
This paper presents an application of the recent relativistic HLLC
approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing
normal component of the magnetic field.
The numerical scheme is validated in two dimensions by investigating the
propagation of axisymmetric jets with toroidal magnetic fields.
The selected jet models show that the HLLC solver yields sharper resolution
of contact and shear waves and better convergence properties over the
traditional HLL approach.Comment: 12 pages, 5 figure
Magnetic acceleration of ultra-relativistic GRB and AGN jets
We present numerical simulations of cold, axisymmetric, magnetically driven
relativistic outflows. The outflows are initially sub-Alfv\'enic and Poynting
flux-dominated, with total--to--rest-mass energy flux ratio up to . To study the magnetic acceleration of jets we simulate flows confined
within a funnel with rigid wall of prescribed shape, which we take to be
(in cylindrical coordinates, with ranging from 1 to 2). This
allows us to eliminate the numerical dissipative effects induced by a free
boundary with an ambient medium. We find that in all cases they converge to a
steady state characterized by a spatially extended acceleration region. For the
jet solutions the acceleration process is very efficient - on the outermost
scale of the simulation more than half of the Poynting flux has been converted
into kinetic energy flux, and the terminal Lorentz factor approached its
maximum possible value (). The acceleration is
accompanied by the collimation of magnetic field lines in excess of that
dictated by the funnel shape. The numerical solutions are generally consistent
with the semi-analytic self-similar jets solutions and the spatially extended
acceleration observed in some astrophysical relativistic jets. In agreement
with previous studies we also find that the acceleration is significantly less
effective for wind solutions suggesting that pulsar winds may remain Poynting
dominated when they reach the termination shock.Comment: 4 pages, 3 figures, HEPRO-2007 Dubli
High--Resolution 3D Simulations of Relativistic Jets
We have performed high-resolution 3D simulations of relativistic jets with
beam flow Lorentz factors up to 7, a spatial resolution of 8 cells per beam
radius, and for up to 75 normalized time units to study the morphology and
dynamics of 3D relativistic jets. Our simulations show that the coherent fast
backflows found in axisymmetric models are not present in 3D models. We further
find that when the jet is exposed to non-axisymmetric perturbations, (i) it
does not display the strong perturbations found for 3D classical hydrodynamic
and MHD jets (at least during the period of time covered by our simulations),
and (ii) it does propagate according to the 1D estimate. Small 3D effects in
the relativistic beam give rise to a lumpy distribution of apparent speeds like
that observed in M87. The beam is surrounded by a boundary layer of high
specific internal energy. The properties of this layer are briefly discussed.Comment: 15 pages, 4 figures. Accepted to be publish in the ApJ Letters.
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