86 research outputs found
Constraints on particle acceleration sites in the Crab Nebula from relativistic MHD simulations
The Crab Nebula is one of the most efficient accelerators in the Galaxy and
the only galactic source showing direct evidence of PeV particles. In spite of
this, the physical process behind such effective acceleration is still a deep
mystery. While particle acceleration, at least at the highest energies, is
commonly thought to occur at the pulsar wind termination shock, the properties
of the upstream flow are thought to be non-uniform along the shock surface, and
important constraints on the mechanism at work come from exact knowledge of
where along this surface particles are being accelerated. Here we use
axisymmetric relativistic MHD simulations to obtain constraints on the
acceleration site(s) of particles of different energies in the Crab Nebula.
Various scenarios are considered for the injection of particles responsible for
synchrotron radiation in the different frequency bands, radio, optical and
X-rays. The resulting emission properties are compared with available data on
the multi wavelength time variability of the inner nebula. Our main result is
that the X-ray emitting particles are accelerated in the equatorial region of
the pulsar wind. Possible implications on the nature of the acceleration
mechanism are discussed.Comment: 12 pages, 7 figures, 2 table
Covariant and 3+1 Equations for Dynamo-Chiral General Relativistic Magnetohydrodynamics
The exponential amplification of initial seed magnetic fields in relativistic
plasmas is a very important topic in astrophysics, from the conditions in the
early Universe to the interior of neutron stars. While dynamo action in a
turbulent plasma is often invoked, in the last years a novel mechanism of
quantum origin has gained increasingly more attention, namely the Chiral
Magnetic Effect (CME). This has been recognized in semi-metals and it is most
likely at work in the quark-gluon plasma formed in heavy-ion collision
experiments, where the highest magnetic fields in nature, up to B~10^18 G, are
produced. This effect is expected to survive even at large hydrodynamical/MHD
scales and it is based on the chiral anomaly due to an imbalance between left-
and right-handed relativistic fermions in the constituent plasma. Such
imbalance leads to an electric current parallel to an external magnetic field,
which is precisely the same mechanism of an alpha-dynamo action in classical
MHD. Here we extend the close parallelism between the chiral and the dynamo
effects to relativistic plasmas and we propose a unified, fully covariant
formulation of the generalized Ohm's law. Moreover, we derive for the first
time the 3+1 general relativistic MHD equations for a chiral plasma both in
flat and curved spacetimes, in view of numerical investigation of the CME in
compact objects, especially magnetars, or of the interplay among the non-ideal
magnetic effects of dynamo, the CME and reconnection.Comment: 11 pages, 3 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
General relativistic magnetohydrodynamics in axisymmetric dynamical spacetimes: the X-ECHO code
Numerical simulations of stellar jets and comparison between synthetic and observed maps: clues to the launch mechanism
High angular resolution spectra obtained with the Hubble Space Telescope
Imaging Spectrograph (HST/STIS) provide rich morphological and kinematical
information about the stellar jet phenomenon, which allows us to test
theoretical models efficiently. In this work, numerical simulations of stellar
jets in the propagation region are executed with the PLUTO code, by adopting
inflow conditions that arise from former numerical simulations of magnetized
outflows, accelerated by the disk-wind mechanism in the launching region. By
matching the two regions, information about the magneto-centrifugal
accelerating mechanism underlying a given astrophysical object can be
extrapolated by comparing synthetic and observed position-velocity diagrams
(PVDs). We show that quite different jets, like those from the young T Tauri
stars DG-Tau and RW-Aur, may originate from the same disk-wind model for
different configurations of the magnetic field at the disk surface. This result
supports the idea that all the observed jets may be generated by the same
mechanism.Comment: 15 pages, 18 figures, accepted for publication by A&
Parametric decay of circularly polarized Alfvén waves: Multidimensional simulations in periodic and open domains
The nonlinear evolution of monochromatic large-amplitude circularly polarized
Alfvén waves subject to the decay instability is studied via numerical
simulations in one, two, and three spatial dimensions.
The asymptotic value of the cross helicity depends strongly on the plasma beta:
in the low beta case multiple decays are observed, with about half of the
energy being transferred to waves propagating in the opposite direction at
lower wave numbers, for each saturation step.
Correspondingly, the other half of the total transverse energy (kinetic and
magnetic) goes into energy carried by the daughter compressive waves and to
the associated shock heating.
In higher beta conditions we find instead that the cross helicity decreases
monotonically with time towards zero, implying an asymptotic balance between
inward and outward Alfvénic modes, a feature similar to the observed
decrease with distance in the solar wind.
Although the instability mainly takes place along the propagation direction,
in the two and three-dimensional case a turbulent cascade occurs also
transverse to the field.
The asymptotic state of density fluctuations appears to be
rather isotropic, whereas a slight preferential cascade
in the transverse direction is seen in magnetic field spectra.
Finally, parametric decay is shown to occur also in a non-periodic domain
with open boundaries, when the mother wave is continuously injected from one
side. In two and three dimensions a strong transverse filamentation
is found at long times, reminiscent of density ray-like
features observed in the extended solar corona and pressure-balanced
structures found in solar wind data
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