2,011 research outputs found
Magnetic field generation in relativistic shocks - An early end of the exponential Weibel instability in electron-proton plasmas
We discuss magnetic field generation by the proton Weibel instability in
relativistic shocks, a situation that applies to the external shocks in the
fireball model for Gamma-ray Bursts, and possibly also to internal shocks. Our
analytical estimates show that the linear phase of the instability ends well
before it has converted a significant fraction of the energy in the proton beam
into magnetic energy: the conversion efficiency is much smaller (of order
m_e/m_p) in electron-proton plasmas than in pair plasmas. We find this estimate
by modelling the plasma in the shock transition zone with a waterbag momentum
distribution for the protons and with a background of hot electrons.
For ultra-relativistic shocks we find that the wavelength of the most
efficient mode for magnetic field generation equals the electron skin depth,
that the relevant nonlinear stabilization mechanism is magnetic trapping, and
that the presence of the hot electrons limits the typical magnetic field
strength generated by this mode so that it does not depend on the energy
content of the protons. We conclude that other processes than the linear Weibel
instability must convert the free energy of the protons into magnetic fields.Comment: 7 pages, 3 figures, accepted for publication in A&
A more accurate numerical scheme for diffusive shock acceleration
We present a more accurate numerical scheme for the calculation of diffusive
shock acceleration of cosmic rays using Stochastic Differential Equations. The
accuracy of this scheme is demonstrated using a simple analytical flow profile
that contains a shock of finite width and a varying diffusivity of the cosmic
rays, where the diffusivity decreases across the shock. We compare the results
for the slope of the momentum distribution with those obtained from a
perturbation analysis valid for finite but small shock width. These
calculations show that this scheme, although computationally more expensive,
provides a significantly better performance than the Cauchy-Euler type schemes
that were proposed earlier in the case where steep gradients in the cosmic ray
diffusivity occur. For constant diffusivity the proposed scheme gives similar
results as the Cauchy-Euler scheme.Comment: 9 pages, 2 figures. Published in MNRA
Ultra-high-energy cosmic ray acceleration by relativistic blast waves
We consider the acceleration of charged particles at the ultra-relativistic
shocks, with Lorentz factors \Gamma_s >> 1 relative to the upstream medium,
arising in relativistic fireball models of gamma-ray bursts (GRBs). We show
that for Fermi-type shock acceleration, particles initially isotropic in the
upstream medium can gain a factor of order \Gamma_s^2 in energy in the first
shock crossing cycle, but that the energy gain factor for subsequent shock
crossing cycles is only of order 2, because for realistic deflection processes
particles do not have time to re-isotropise upstream before recrossing the
shock.
We evaluate the maximum energy attainable and the efficiency of this process,
and show that for a GRB fireball expanding into a typical interstellar medium,
these exclude the production of ultra-high-energy cosmic rays (UHECRs), with
energies in the range 10^{18.5} - 10^{20.5} eV, by the blast wave. We propose,
however, that in the context of neutron star binaries as the progenitors of
GRBs, relativistic ions from the pulsar wind bubbles produced by these systems
could be accelerated by the blast wave. We show that if the known binary
pulsars are typical, the maximum energy, efficiency, and spectrum in this case
can account for the observed population of UHECRs.Comment: Accepted for MNRAS (Letters), with minor revisions. LaTeX, 5 pages,
uses mn.st
Evolution of Magnetic Fields in Supernova Remnants
Supernova remnants (SNR) are now widely believed to be a source of cosmic
rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate
CRs to sufficiently high energies need to be much higher than can result from
compression of the circumstellar medium (CSM) by a factor 4, as is the case in
strong shocks. Non-thermal synchrotron maps of these regions indicate that
indeed the magnetic field is much stronger, and for young SNRs has a dominant
radial component while for old SNRs it is mainly toroidal. How these magnetic
fields get enhanced, or why the field orientation is mainly radial for young
remnants, is not yet fully understood. We use an adaptive mesh refinement MHD
code, AMRVAC, to simulate the evolution of supernova remnants and to see if we
can reproduce a mainly radial magnetic field in early stages of evolution. We
follow the evolution of the SNR with three different configurations of the
initial magnetic field in the CSM: an initially mainly toroidal field, a
turbulent magnetic field, and a field parallel to the symmetry axis. Although
for the latter two topologies a significant radial field component arises at
the contact discontinuity due to the Rayleigh-Taylor instability, no radial
component can be seen out to the forward shock. Ideal MHD appears not
sufficient to explain observations. Possibly a higher compression ratio and
additional turbulence due to dominant presence of CRs can help us to better
reproduce the observations in future studies.Comment: 5 pages, 3 figures. To appear in conference proceedings of "Magnetic
Fields in the Universe II" (2008), RevMexA
Nonthermal X-ray emission from young Supernova Remnants
The cosmic-ray spectrum up to the knee ( eV) is attributed to
acceleration processes taking place at the blastwaves which bound supernova
remnants. Theoretical predictions give a similar estimate for the maximum
energy which can be reached at supernova remnant shocks by particle
acceleration. Electrons with energies of the order eV should
give a nonthermal X-ray component in young supernova remnants. Recent
observations of SN1006 and G347.3-0.5 confirm this prediction. We present a
method which uses hydrodynamical simulations to describe the evolution of a
young remnant. These results are combined with an algorithm which
simultaneously calculates the associated particle acceleration. We use the test
particle approximation, which means that the back-reaction on the dynamics of
the remnant by the energetic particles is neglected. We present synchrotron
maps in the X-ray domain, and present spectra of the energies of the electrons
in the supernova remnant. Some of our results can be compared directly with
earlier semi-analytical work on this subject by Reynolds [1].Comment: 4 pages, 2 figures, contribution proceedings of poster presented at
the 11th Annual Astrophysics Conference in Maryland, to appear in Young
Supernova Remnants, ed. by S. S. Holt and U. Hwang (AIP
Branching on multi-aggregated variables
open5siopenGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, DomenicoGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, Domenic
Pulsar wind nebulae in supernova remnants
A spherically symmetric model is presented for the interaction of a pulsar
wind with the associated supernova remnant. This results in a pulsar wind
nebula whose evolution is coupled to the evolution of the surrounding supernova
remnant. This evolution can be divided in three stages. The first stage is
characterised by a supersonic expansion of the pulsar wind nebula into the
freely expanding ejecta of the progenitor star. In the next stage the pulsar
wind nebula is not steady; the pulsar wind nebula oscillates between
contraction and expansion due to interaction with the reverse shock of the
supernova remnant: reverberations which propagate forward and backward in the
remnant. After the reverberations of the reverse shock have almost completely
vanished and the supernova remnant has relaxed to a Sedov solution, the
expansion of the pulsar wind nebula proceeds subsonically. In this paper we
present results from hydrodynamical simulations of a pulsar wind nebula through
all these stages in its evolution. The simulations were carried out with the
Versatile Advection Code.Comment: 10 pages, 9 figures, submitted to Astronomy and Astrophysic
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