143 research outputs found
The inner knot of the Crab nebula
We model the inner knot of the Crab Nebula as a synchrotron emission coming
from the non-spherical MHD termination shock of relativistic pulsar wind. The
post-shock flow is mildly relativistic; as a result the Doppler-beaming has a
strong impact on the shock appearance. The model can reproduce the knot
location, size, elongation, brightness distribution, luminosity and
polarization provided the effective magnetization of the section of the pulsar
wind producing the knot is low, . In the striped wind model,
this implies that the striped zone is rather wide, with the magnetic
inclination angle of the Crab pulsar ; this agrees with the
previous model-dependent estimate based on the gamma-ray emission of the
pulsar. We conclude that the tiny knot is indeed a bright spot on the surface
of a quasi-stationary magnetic relativistic shock and that this shock is a site
of efficient particle acceleration. On the other hand, the deduced low
magnetization of the knot plasma implies that this is an unlikely site for the
Crab's gamma-ray flares, if they are related to the fast relativistic magnetic
reconnection events.Comment: 16 pages, 17 figure
Test particles in relativistic resistive magnetohydrodynamics
The Black Hole Accretion Code (BHAC) has recently been extended with the
ability to evolve charged test particles according to the Lorentz force within
resistive relativistic magnetohydrodynamics simulations. We apply this method
to evolve particles in a reconnecting current sheet that forms due to the
coalescence of two magnetic flux tubes in 2D Minkowski spacetime. This is the
first analysis of charged test particle evolution in resistive relativistic
magnetohydrodynamics simulations. The energy distributions of an ensemble of
100.000 electrons are analyzed, as well as the acceleration of particles in the
plasmoids that form in the reconnection layer. The effect of the Lundquist
number, magnetization, and plasma- on the particle energy distribution
is explored for a range of astrophysically relevant parameters. We find that
electrons accelerate to non-thermal energies in the thin current sheets in all
cases. We find two separate acceleration regimes: An exponential increase of
the Lorentz factor during the island coalescence where the acceleration depends
linearly on the resistivity and a nonlinear phase with high variability. These
results are relevant for determining energy distributions and acceleration
sites obtaining radiation maps in large-scale magnetohydrodynamics simulations
of black hole accretion disks and jets.Comment: Matching accepted version in J. Phys.: Conf. Ser. Astronum 2018
Proceeding
Magnetically inspired explosive outflows from neutron-star mergers
Binary neutron-star mergers have long been associated with short-duration
gamma-ray bursts (GRBs). This connection was confirmed with the first
coincident detection of gravitational waves together with electromagnetic
radiation from GW170817. The basic paradigm for short-duration GRBs includes an
ultra-relativistic jet, but the low-luminosity prompt emission together with
follow-up radio and X-ray observations have hinted that this picture may be
different in the case of GW170817. In particular, it has been proposed that
large amounts of the magnetic energy that is amplified after the merger, can be
released when the remnant collapses to a black hole, giving rise to a
quasi-spherical explosion impacting on the merger ejecta. Through numerical
simulations we investigate this scenario for a range of viewing angles,
injected energies and matter densities at the time of the collapse. Depending
on the magnitude of the energy injection and the remnant density, we find two
types of outflows: one with a narrow relativistic core and one with a
wide-angle, but mildly relativistic outflow. Furthermore, very wide outflows
are possible, but require energy releases in excess of 10^52 erg.Comment: matched published version ApJ Letter
Relativistic resistive magnetohydrodynamic reconnection and plasmoid formation in merging flux tubes
We apply the general relativistic resistive magnetohydrodynamics code {\tt
BHAC} to perform a 2D study of the formation and evolution of a reconnection
layer in between two merging magnetic flux tubes in Minkowski spacetime.
Small-scale effects in the regime of low resistivity most relevant for dilute
astrophysical plasmas are resolved with very high accuracy due to the extreme
resolutions obtained with adaptive mesh refinement. Numerical convergence in
the highly nonlinear plasmoid-dominated regime is confirmed for a sweep of
resolutions. We employ both uniform resistivity and non-uniform resistivity
based on the local, instantaneous current density. For uniform resistivity we
find Sweet-Parker reconnection, from down to ,
for a reference case of magnetisation and plasma-.
{For uniform resistivity the tearing mode is recovered,
resulting in the formation of secondary plasmoids. The plasmoid instability
enhances the reconnection rate to compared to for .} For non-uniform resistivity with a base
level and an enhanced current-dependent resistivity in the
current sheet, we find an increased reconnection rate of . The influence of the magnetisation and the plasma- is
analysed for cases with uniform resistivity and
in a range and in regimes that are applicable for black hole accretion disks and jets. The
plasmoid instability is triggered for Lundquist numbers larger than a critical
value of .Comment: Matching accepted version in MNRA
Synchrotron radiation of self-collimating relativistic MHD jets
The goal of this paper is to derive signatures of synchrotron radiation from
state-of-the-art simulation models of collimating relativistic
magnetohydrodynamic (MHD) jets featuring a large-scale helical magnetic field.
We perform axisymmetric special relativistic MHD simulations of the jet
acceleration region using the PLUTO code. The computational domain extends from
the slow magnetosonic launching surface of the disk up to 6000^2 Schwarzschild
radii allowing to reach highly relativistic Lorentz factors. The Poynting
dominated disk wind develops into a jet with Lorentz factors of 8 and is
collimated to 1 degree. In addition to the disk jet, we evolve a thermally
driven spine jet, emanating from a hypothetical black hole corona. Solving the
linearly polarized synchrotron radiation transport within the jet, we derive
VLBI radio and (sub-) mm diagnostics such as core shift, polarization
structure, intensity maps, spectra and Faraday rotation measure (RM), directly
from the Stokes parameters. We also investigate depolarization and the
detectability of a lambda^2-law RM depending on beam resolution and observing
frequency. We find non-monotonic intrinsic RM profiles which could be detected
at a resolution of 100 Schwarzschild radii. In our collimating jet geometry,
the strict bi-modality in polarization direction (as predicted by Pariev et
al.) can be circumvented. Due to relativistic aberration, asymmetries in the
polarization vectors across the jet can hint to the spin direction of the
central engine.Comment: Submitted to Ap
Generalized, energy-conserving numerical simulations of particles in general relativity. II. Test particles in electromagnetic fields and GRMHD
Direct observations of compact objects, in the form of radiation spectra,
gravitational waves from VIRGO/LIGO, and forthcoming direct imaging, are
currently one of the primary source of information on the physics of plasmas in
extreme astrophysical environments. The modeling of such physical phenomena
requires numerical methods that allow for the simulation of microscopic plasma
dynamics in presence of both strong gravity and electromagnetic fields. In
Bacchini et al. (2018) we presented a detailed study on numerical techniques
for the integration of free geodesic motion. Here we extend the study by
introducing electromagnetic forces in the simulation of charged particles in
curved spacetimes. We extend the Hamiltonian energy-conserving method presented
in Bacchini et al. (2018) to include the Lorentz force and we test its
performance compared to that of standard explicit Runge-Kutta and implicit
midpoint rule schemes against analytic solutions. Then, we show the application
of the numerical schemes to the integration of test particle trajectories in
general relativistic magnetohydrodynamic (GRMHD) simulations, by modifying the
algorithms to handle grid-based electromagnetic fields. We test this approach
by simulating ensembles of charged particles in a static GRMHD configuration
obtained with the Black Hole Accretion Code (BHAC)
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