49 research outputs found
Internal shocks in relativistic outflows: collisions of magnetized shells
(Abridged): We study the collision of magnetized irregularities (shells) in
relativistic outflows in order to explain the origin of the generic
phenomenology observed in the non-thermal emission of both blazars and
gamma-ray bursts. We focus on the influence of the magnetic field on the
collision dynamics, and we further investigate how the properties of the
observed radiation depend on the strength of the initial magnetic field and on
the initial internal energy density of the flow. The collisions of magnetized
shells and the radiation resulting from these collisions are calculated using
the 1D relativistic magnetohydrodynamics code MRGENESIS. The interaction of the
shells with the external medium prior to their collision is also determined
using an exact solver for the corresponding 1D relativistic magnetohydrodynamic
Riemann problem. Our simulations show that two magnetization parameters - the
ratio of magnetic energy density and thermal energy density, \alpha_B, and the
ratio of magnetic energy density and mass-energy density, \sigma - play an
important role in the pre-collision phase, while the dynamics of the collision
and the properties of the light curves depend mostly on the magnetization
parameter \sigma. The interaction of the shells with the external medium
changes the flow properties at their edges prior to the collision. For
sufficiently dense shells moving at large Lorentz factors (\simgt 25) these
properties depend only on the magnetization parameter \sigma. Internal shocks
in GRBs may reach maximum efficiencies of conversion of kinetic into thermal
energy between 6% and 10%, while in case of blazars, the maximum efficiencies
are \sim 2%.Comment: 17 pages, 18 figures. 2 new references have been added. Accepted for
publication in Astronomy and Astrophysic
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
The magnetohydrodynamic instability of current-carrying jets
Magnetohydrodynamic instabilities can be responsible for the formation of
structures with various scales in astrophysical jets. We consider the stability
properties of jets containing both the azimuthal and axial field of subthermal
strength. A magnetic field with complex topology in jets is suggested by
theoretical models and is consistent with recent observations. Stability is
discussed by means of a linear analysis of the ideal magnetohydrodynamic
equations. We argue that in azimuthal and axial magnetic fields the jet is
always unstable to non-axisymmetric perturbations. Stabilization does not occur
even if the strengths of these field components are comparable. If the axial
field is weaker than the azimuthal one, instability occurs for perturbations
with any azimuthal wave number , and the growth rate reaches a saturation
value for low values of . If the axial field is stronger than the toroidal
one, the instability shows for perturbations with relatively high .Comment: 9 pages, 9 figures, to appear on A&
Equation of State in Relativistic Magnetohydrodynamics: variable versus constant adiabatic index
The role of the equation of state for a perfectly conducting, relativistic
magnetized fluid is the main subject of this work. The ideal constant
-law equation of state, commonly adopted in a wide range of
astrophysical applications, is compared with a more realistic equation of state
that better approximates the single-specie relativistic gas. The paper focus on
three different topics. First, the influence of a more realistic equation of
state on the propagation of fast magneto-sonic shocks is investigated. This
calls into question the validity of the constant -law equation of state
in problems where the temperature of the gas substantially changes across
hydromagnetic waves. Second, we present a new inversion scheme to recover
primitive variables (such as rest-mass density and pressure) from conservative
ones that allows for a general equation of state and avoids catastrophic
numerical cancellations in the non-relativistic and ultrarelativistic limits.
Finally, selected numerical tests of astrophysical relevance (including
magnetized accretion flows around Kerr black holes) are compared using
different equations of state. Our main conclusion is that the choice of a
realistic equation of state can considerably bear upon the solution when
transitions from cold to hot gas (or viceversa) are present. Under these
circumstances, a polytropic equation of state can significantly endanger the
solution.Comment: 14 pages, 14 figure
On the dynamic efficiency of internal shocks in magnetized relativistic outflows
We study the dynamic efficiency of conversion of kinetic-to-thermal/magnetic
energy of internal shocks in relativistic magnetized outflows. We model
internal shocks as being caused by collisions of shells of plasma with the same
energy flux and a non-zero relative velocity. The contact surface, where the
interaction between the shells takes place, can break up either into two
oppositely moving shocks (in the frame where the contact surface is at rest),
or into a reverse shock and a forward rarefaction. We find that for moderately
magnetized shocks (magnetization ), the dynamic efficiency in
a single two-shell interaction can be as large as 40%. Thus, the dynamic
efficiency of moderately magnetized shocks is larger than in the corresponding
unmagnetized two-shell interaction. If the slower shell propagates with a
sufficiently large velocity, the efficiency is only weakly dependent on its
Lorentz factor. Consequently, the dynamic efficiency of shell interactions in
the magnetized flow of blazars and gamma-ray bursts is effectively the same.
These results are quantitatively rather independent on the equation of state of
the plasma. The radiative efficiency of the process is expected to be a
fraction of the estimated dynamic one, the exact value of
depending on the particularities of the emission processes which radiate away
the thermal or magnetic energy of the shocked states.Comment: Accepted for publication in MNRAS. 8 pages, 6 figures. The definitive
version is available at http://www.blackwell-synergy.co
An HLLC Solver for Relativistic Flows -- II. Magnetohydrodynamics
An approximate Riemann solver for the equations of relativistic
magnetohydrodynamics (RMHD) is derived. The HLLC solver, originally developed
by Toro, Spruce and Spears, generalizes the algorithm described in a previous
paper (Mignone & Bodo 2004) to the case where magnetic fields are present. The
solution to the Riemann problem is approximated by two constant states bounded
by two fast shocks and separated by a tangential wave. The scheme is
Jacobian-free, in the sense that it avoids the expensive characteristic
decomposition of the RMHD equations and it improves over the HLL scheme by
restoring the missing contact wave.
Multidimensional integration proceeds via the single step, corner transport
upwind (CTU) method of Colella, combined with the contrained tranport (CT)
algorithm to preserve divergence-free magnetic fields. The resulting numerical
scheme is simple to implement, efficient and suitable for a general equation of
state. The robustness of the new algorithm is validated against one and two
dimensional numerical test problems.Comment: 17 pages, 12 figure
Risco a partir do coeficiente beta do Modelo CAPM e sua relação com o Dividend Yield das empresas do ISE-Índice de Sustentabilidade Empresarial- da BMFBovespa do período 2004/2015.
Este estudo teve como objetivo identificar se ativos com maior risco mensurável pelo coeficiente beta possuem dividend yield diferente das com menor risco. A amostra deste estudo é representada pelos ativos de empresas de capital aberto com listagem no índice de sustentabilidade empresarial (ISE), considerados os 23 ativos que possuem dados quanto ao retorno diário em pelo menos 90% de cada um dos trimestres analisados no período que corresponde a 01/01/2004 até 01/01/2015. A pesquisa se caracteriza como descritiva, documental e de caráter quantitativo. Após a separação dos ativos em três grupos distintos, considerando a média dos 44 trimestres estudados, os resultados indicam que o grupo 1, composto pelos ativos com menor coeficiente Beta, possuem em média um dividend yield maior em 30,44% se comparado ao grupo 2, e 126% maior do que a média do grupo 3, que é composto pelos ativos com maior coeficiente Beta.
Multiwavelength afterglow light curves from magnetized GRB flows
We use high-resolution relativistic MHD simulations coupled with a radiative
transfer code to compute multiwavelength afterglow light curves of magnetized
ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The
aim of our study is to determine how the magnetization of the ejecta at large
distance from the central engine influences the afterglow emission, and to
assess whether observations can be reliably used to infer the strength of the
magnetic field. We find that, for typical parameters of the ejecta, the
emission from the reverse shock peaks for magnetization of the flow, and that it is greatly suppressed for higher . The
emission from the forward shock shows an achromatic break shortly after the end
of the burst marking the onset of the self-similar evolution of the blast wave.
Fitting the early afterglow of GRB 990123 and 090102 with our numerical models
we infer respective magnetizations of and for these bursts. We argue that the lack of observed reverse shock
emission from the majority of the bursts can be understood if \sigma_0
\simmore 0.1, since we obtain that the luminosity of the reverse shock
decreases significantly for . For ejecta with \sigma_0
\simmore 0.1 our models predict that there is sufficient energy left in the
magnetic field, at least during an interval of ~10 times the burst duration, to
produce a substantial emission if the magnetic energy can be dissipated (for
instance, due to resistive effects) and radiated away.Comment: 9 pages, 9 figures. Submitted to MNRAS
General relativistic simulations of pasive-magneto-rotational core collapse with microphysics
This paper presents results from axisymmetric simulations of
magneto-rotational stellar core collapse to neutron stars in general relativity
using the passive field approximation for the magnetic field. These simulations
are performed using a new general relativistic numerical code specifically
designed to study this astrophysical scenario. The code is based on the
conformally-flat approximation of Einstein's field equations and conservative
formulations of the magneto-hydrodynamics equations. The code has been recently
upgraded to incorporate a tabulated, microphysical equation of state and an
approximate deleptonization scheme. This allows us to perform the most
realistic simulations of magneto-rotational core collapse to date, which are
compared with simulations employing a simplified (hybrid) equation of state,
widely used in the relativistic core collapse community. Furthermore,
state-of-the-art (unmagnetized) initial models from stellar evolution are used.
In general, stellar evolution models predict weak magnetic fields in the
progenitors, which justifies our simplification of performing the computations
under the approach that we call the passive field approximation for the
magnetic field. Our results show that for the core collapse models with
microphysics the saturation of the magnetic field cannot be reached within
dynamical time scales by winding up the poloidal magnetic field into a toroidal
one. We estimate the effect of other amplification mechanisms including the
magneto-rotational instability (MRI) and several types of dynamos.Comment: 25 pages, 15 figures, accepted for publication in Astronomy &
Astrophysics July 31, 2007. Added 1 figure and a new subsectio
Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution
(Abridged) We aim to quantitatively understand the dynamical effect and
observational signatures of magnetization of the GRB ejecta on the onset of the
afterglow. We perform ultrahigh-resolution one-dimensional relativistic MHD
simulations of the interaction of a radially expanding, magnetized ejecta with
the interstellar medium. The need of ultrahigh numerical resolution derives
from the extreme jump conditions in the region of interaction between the
ejecta and the circumburst medium. We study the evolution of an
ultrarelativistic shell all the way to a the self-similar asymptotic phase. Our
simulations show that the complete evolution can be characterized in terms of
two parameters, namely, the \xi parameter introduced by Sari & Piran (1995) and
the magnetization \sigma_0. We exploit this property by producing numerical
models where the shell Lorentz factor is \gamma_0 ~ tens and rescaling the
results to arbitrarily large \gamma_0. We find that the reverse shock is
typically very weak or absent for ejecta characterized by \sigma_0 >~ 1. The
onset of the forward shock emission is strongly affected by the magnetization.
On the other hand, the magnetic energy of the shell is transfered to the
external medium on a short timescale (~several times the duration of the
burst). The later forward shock emission does not contain information for the
initial magnetization of the flow. The asymptotic evolution of strongly
magnetized shells, after they have suffred a substantial deceleration,
resembles that of hydrodynamic shells, i.e., they fully enter in the
Blandford-McKee self-similar regime.Comment: 14 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic