788 research outputs found
Relativistic particle transport in extragalactic jets: I. Coupling MHD and kinetic theory
Multidimensional magneto-hydrodynamical (MHD) simulations coupled with
stochastic differential equations (SDEs) adapted to test particle acceleration
and transport in complex astrophysical flows are presented. The numerical
scheme allows the investigation of shock acceleration, adiabatic and radiative
losses as well as diffusive spatial transport in various diffusion regimes. The
applicability of SDEs to astrophysics is first discussed in regards to the
different regimes and the MHD code spatial resolution. The procedure is then
applied to 2.5D MHD-SDE simulations of kilo-parsec scale extragalactic jets.
The ability of SDE to reproduce analytical solutions of the
diffusion-convection equation for electrons is tested through the incorporation
of an increasing number of effects: shock acceleration, spatially dependent
diffusion coefficients and synchrotron losses. The SDEs prove to be efficient
in various shock configuration occurring in the inner jet during the
development of the Kelvin-Helmholtz instability. The particle acceleration in
snapshots of strong single and multiple shock acceleration including realistic
spatial transport is treated. In chaotic magnetic diffusion regime, turbulence
levels around are found to
be the most efficient to enable particles to reach the highest energies. The
spectrum, extending from 100 MeV to few TeV (or even 100 TeV for fast flows),
does not exhibit a power-law shape due to transverse momentum dependent
escapes. Out of this range, the confinement is not so efficient and the
spectrum cut-off above few hundreds of GeV, questioning the Chandra
observations of X-ray knots as being synchrotron radiation. The extension to
full time dependent simulations to X-ray extragalactic jets is discussed.Comment: Astronomy & Astrophysics (in press), 18 page
Cosmic Ray propagation in sub-Alfvenic magnetohydrodynamic turbulence
This work has the main objective to provide a detailed investigation of
cosmic ray propagation in magnetohydrodynamic turbulent fields generated by
forcing the fluid velocity field at large scales. It provides a derivation of
the particle mean free path dependences in terms of the turbulence level
described by the Alfv\'enic Mach number and in terms of the particle rigidity.
We use an upgrade version of the magnetohydrodynamic code {\tt RAMSES} which
includes a forcing module and a kinetic module and solve the Lorentz equation
for each particle. The simulations are performed using a 3 dimension periodical
box in the test-particle and magnetostatic limits. The forcing module is
implemented using an Ornstein-Uhlenbeck process. An ensemble average over a
large number of particle trajectories is applied to reconstruct the particle
mean free paths. We derive the cosmic ray mean free paths in terms of the
Alfv\'enic Mach numbers and particle reduced rigidities in different turbulence
forcing geometries. The reduced particle rigidity is where
is the particle Larmor radius and is the simulation box length related to
the turbulence coherence or injection scale by . We
have investigated with a special attention compressible and solenoidal forcing
geometries. We find that compressible forcing solutions are compatible with the
quasi-linear theory or more advanced non-linear theories which predict a
rigidity dependence as or . Solenoidal forcing
solutions at least at low or moderate Alfv\'enic numbers are not compatible
with the above theoretical expectations and require more refined arguments to
be interpreted. It appears especially for Alfv\'enic Mach numbers close to one
that the wandering of field lines controls perpendicular mean free path
solutions whatever the forcing geometry.Comment: 15 pages, 18 figures. Accepted for publication in section 2.
Astrophysical processes of Astronomy and Astrophysic
MARCOS, a numerical tool for the simulation of multiple time-dependent non-linear diffusive shock acceleration
We present a new code aimed at the simulation of diffusive shock acceleration
(DSA), and discuss various test cases which demonstrate its ability to study
DSA in its full time-dependent and non-linear developments. We present the
numerical methods implemented, coupling the hydrodynamical evolution of a
parallel shock (in one space dimension) and the kinetic transport of the
cosmic-rays (CR) distribution function (in one momentum dimension), as first
done by Falle. Following Kang and Jones and collaborators, we show how the
adaptive mesh refinement technique (AMR) greatly helps accommodating the
extremely demanding numerical resolution requirements of realistic (Bohm-like)
CR diffusion coefficients. We also present the paral lelization of the code,
which allows us to run many successive shocks at the cost of a single shock,
and thus to present the first direct numerical simulations of linear and
non-linear multiple DSA, a mechanism of interest in various astrophysical
environments such as superbubbles, galaxy clusters and early cosmological
flows.Comment: accepted for publication in MNRAS by the Royal Astronomical Society
and Blackwell Publishin
Non-resonant magnetohydrodynamics streaming instability near magnetized relativistic shocks
We present in this paper both a linear study and numerical relativistic MHD
simulations of the non-resonant streaming instability occurring in the
precursor of relativistic shocks. In the shock front restframe, we perform a
linear analysis of this instability in a likely configuration for
ultra-relativistic shock precursors. This considers magneto-acoustic waves
having a wave vector perpendicular to the shock front and the large scale
magnetic field. Our linear analysis is achieved without any assumption on the
shock velocity and is thus valid for all velocity regimes. In order to check
our calculation, we also perform relativistic MHD simulations describing the
propagation of the aforementioned magneto-acoustic waves through the shock
precursor. The numerical calculations confirm our linear analysis, which
predicts that the growth rate of the instability is maximal for
ultra-relativistic shocks and exhibits a wavenumber dependence . Our numerical simulations also depict the saturation regime of the
instability where we show that the magnetic amplification is moderate but
nevertheless significant (). This latter fact may explain
the presence of strong turbulence in the vicinity of relativistic magnetized
shocks. Our numerical approach also introduces a convenient means to handle
isothermal (ultra-)relativistic MHD conditions.Comment: 14 pages, 6 figures, MNRAS (in press
Transport of positrons in the interstellar medium
This work investigates some aspects of the transport of low-energy positrons
in the interstellar medium (ISM). We consider resonance interactions with
magnetohydrodynamic waves above the resonance threshold. Below the threshold,
collisions take over and deflect positrons in their motion parallel to
magnetic-field lines. Using Monte-Carlo simulations, we model the propagation
and energy losses of positrons in the different phases of the ISM until they
annihilate. We suggest that positrons produced in the disk by an old population
of stars, with initial kinetic energies below 1 MeV, and propagating in the
spiral magnetic field of the disk, can probably not penetrate the Galactic
bulge.Comment: 4 pages, 3 figures, accepted for publication in the proceeding of the
6th INTEGRAL worksho
Radiation processes around accreting black holes
Accreting sources such as AGN, X-ray binaries or gamma-ray bursts are known
to be strong, high energy emitters. The hard emission is though to originate
from plasmas of thermal and/or non-thermal high energy particles. Not only does
this emission allow to probe the unique properties of the matter in an extreme
environment, but it also has a crucial backreaction on the energetics and the
dynamics of the emitting medium itself. Understanding interactions between
radiation and matter has become a key issue in the modelling of high energy
sources. Although most cross sections are well known, they are quite complex
and the way all processes couple non-linearly is still an open issue.
We present a new code that solves the local, kinetic evolution equations for
distributions of electrons, positrons and photons, interacting by radiation
processes such as self-absorbed synchrotron and brems-strahlung radiation,
Compton scattering, pair production/annihilation, and by Coulomb collisions.
The code is very general and aimed to modelled various high energy sources. As
an application, we study the spectral states of X-ray binaries, including
thermalization by Coulomb collisions and synchrotron self-absorption. It is
found that the low-hard and high-soft states can be modelled with different
illumination but the same non-thermal acceleration mechanism.Comment: 4 pages, 2 figures, proceedings of the SF2A conference 200
Cosmic-ray acceleration and gamma-ray signals from radio supernovae
In this work the efficiency of particle acceleration at the forward shock
right after the SN outburst for the particular case of the well-known SN 1993J
is analyzed. Plasma instabilities driven by the energetic particles accelerated
at the shock front grow over intraday timescales and drive a fast amplification
of the magnetic field at the shock, that can explain the magnetic field
strengths deduced from the radio monitoring of the source. The maximum particle
energy is found to reach 1-10 PeV depending on the instability dominating the
amplification process. We derive the time dependent particle spectra and the
associated hadronic signatures of secondary particles arising from proton
proton interactions. We find that the Cherenkov Telescope Array (CTA) should
easily detect objects like SN 1993J in particular above 1 TeV, while current
generation of Cherenkov telescopes such as H.E.S.S. could only marginally
detect such events. The gamma-ray signal is found to be heavily absorbed by
pair production process during the first week after the outburst. We predict a
low neutrino flux above 10 TeV, implying a detectability horizon with a
KM3NeT-type telescope of 1 Mpc only. We finally discuss the essential
parameters that control the particle acceleration and gamma-ray emission in
other type of SNe.Comment: 7 pages, 3 figures (Note: conflict of .sty file version explains the
problems with journal title and the abstract, apologies for any
inconvenience). Appears as Nuclear Physics B Proceedings Supplement 2014
Proceedings of the workshop "Cosmic Ray Origin beyond the standard models",
San Vito (2014), ed. by O.Tibolla, L. Drur
Cosmic-ray acceleration in young protostars
The main signature of the interaction between cosmic rays and molecular
clouds is the high ionisation degree. This decreases towards the densest parts
of a cloud, where star formation is expected, because of energy losses and
magnetic effects. However recent observations hint to high levels of ionisation
in protostellar systems, therefore leading to an apparent contradiction that
could be explained by the presence of energetic particles accelerated within
young protostars. Our modelling consists of a set of conditions that has to be
satisfied in order to have an efficient particle acceleration through the
diffusive shock acceleration mechanism. We find that jet shocks can be strong
accelerators of protons which can be boosted up to relativistic energies.
Another possibly efficient acceleration site is located at protostellar
surfaces, where shocks caused by impacting material during the collapse phase
are strong enough to accelerate protons. Our results demonstrate the
possibility of accelerating particles during the early phase of a
proto-Solar-like system and can be used as an argument to support available
observations. The existence of an internal source of energetic particles can
have a strong and unforeseen impact on the star and planet formation process as
well as on the formation of pre-biotic molecules.Comment: Accepted by Astronomy and Astrophysic
Shape and evolution of wind-blown bubbles of massive stars: on the effect of the interstellar magnetic field
The winds of massive stars create large (>10 pc) bubbles around their
progenitors. As these bubbles expand they encounter the interstellar coherent
magnetic field which, depending on its strength, can influence the shape of the
bubble. We wish to investigate if, and how much, the interstellar magnetic
field can contribute to the shape of an expanding circumstellar bubble around a
massive star. We use the MPI-AMRVAC code to make magneto-hydrodynamical
simulations of bubbles, using a single star model, combined with several
different field strengths: B = 5, 10, and 20 muG for the interstellar magnetic
field. This covers the typical field strengths of the interstellar magnetic
fields found in the galactic disk and bulge. Furthermore, we present two
simulations that include both a 5 muG interstellar magnetic field and a 10,000
K interstellar medium and two different ISM densities to demonstrate how the
magnetic field can combine with other external factors to influence the
morphology of the circumstellar bubbles. Our results show that low magnetic
fields, as found in the galactic disk, inhibit the growth of the circumstellar
bubbles in the direction perpendicular to the field. As a result, the bubbles
become ovoid, rather than spherical. Strong interstellar fields, such as
observed for the galactic bulge, can completely stop the expansion of the
bubble in the direction perpendicular to the field, leading to the formation of
a tube-like bubble. When combined with a warm, high-density ISM the bubble is
greatly reduced in size, causing a dramatic change in the evolution of
temporary features inside the bubble. The magnetic field of the interstellar
medium can affect the shape of circumstellar bubbles. This effect may have
consequences for the shape and evolution of circumstellar nebulae and supernova
remnants, which are formed within the main wind-blown bubble.Comment: Proposed for acceptance for publication in Astronomy & Astrophysics.
The published version will contain animations of each simulatio
- âŠ