247 research outputs found
Highly magnetized region in pulsar wind nebulae and origin of the Crab gamma-ray flares
The recently discovered gamma-ray flares from the Crab nebula are generally
attributed to the magnetic energy release in a highly magnetized region within
the nebula. I argue that such a region naturally arises in the polar region of
the inner nebula. In pulsar winds, efficient dissipation of the Poynting flux
into the plasma energy occur only in the equatorial belt where the energy is
predominantly transferred by alternating fields. At high latitudes, the pulsar
wind remains highly magnetized therefore the termination shock in the polar
region is weak and the postshock flow remains relativistic. I study the
structure of this flow and show that the flow at first expands and decelerates
and then it converges and accelerates. In the converging part of the flow, the
kink instability triggers the magnetic dissipation. The energy release zone
occurs at the base of the observed jet. A specific turbulence of
relativistically shrinking magnetic loops efficiently accelerates particles so
that the synchrotron emission in the hundreds MeV band, both persistent and
flaring, comes from this site.Comment: Submitted to MNRA
Fast magnetosonic waves in pulsar winds
Fast magnetosonic waves in a magnetically-dominated plasma are investigated.
In the pulsar wind, these waves may transport a significant fraction of the
energy flux. It is shown that the nonlinear steepening and subsequent formation
of multiple shocks is a viable mechanism for the wave dissipation in the pulsar
wind. The wave dissipation both in the free pulsar wind and beyond the wind
termination shock is considered.Comment: 8 pages, 1 eps figure, to appear in MNRA
On the relativistic magnetic reconnection
Reconnection of the magnetic lines of force is considered in case the
magnetic energy exceeds the rest energy of the matter. It is shown that the
classical Sweet-Parker and Petschek models are generalized straightforwardly to
this case and the reconnection rate may be estimated by substituting the Alfven
velocity in the classical formulas by the speed of light. The outflow velocity
in the Sweet-Parker configuration is mildly relativistic. In the Petschek
configuration, the outflow velocity is ultrarelativistic whereas the angle
between the slow shocks is very small. Due to the strong compression, the
plasma outflow in the Petschek configuration may become strongly magnetized if
the reconnecting fields are not exactly antiparallel.Comment: Accepted by MNRA
Transformation of the Poynting flux into the kinetic energy in relativistic jets
The acceleration of relativistic jets from the Poynting to the matter
dominated stage is considered. The are generally two collimation regimes, which
we call equilibrium and non-equilibrium, correspondingly. In the first regime,
the jet is efficiently accelerated till the equipartition between the kinetic
and electro-magnetic energy. We show that after the equilibrium jet ceases to
be Poynting dominated, the ratio of the electro-magnetic to the kinetic energy
decreases only logarithmically so that such jets become truly matter dominated
only at extremely large distances. Non-equilibrium jets remain generally
Poynting dominated till the logarithmically large distances. In the only case
when a non-equilibrium jet is accelerated till the equipartition level, we
found that the flow is not continued to the infinity but is focused towards the
axis at a finite distance from the origin.Comment: Submitted to MNRAS Minor changes in the Conclusion
Reconnection in pulsar winds
The spin-down power of a pulsar is thought to be carried away in an MHD wind
in which, at least close to the star, the energy transport is dominated by
Poynting flux. The pulsar drives a low-frequency wave in this wind, consisting
of stripes of toroidal magnetic field of alternating polarity, propagating in a
region around the equatorial plane. The current implied by this configuration
falls off more slowly with radius than the number of charged particles
available to carry it, so that the MHD picture must, at some point, fail.
Recently, magnetic reconnection in such a structure has been shown to
accelerate the wind significantly. This reduces the magnetic field in the
comoving frame and, consequently, the required current, enabling the solution
to extend to much larger radius. This scenario is discussed and, for the Crab
Nebula, the range of validity of the MHD solution is compared with the radius
at which the flow appears to terminate. For sufficiently high particle
densities, it is shown that a low frequency entropy wave can propagate out to
the termination point. In this case, the "termination shock" itself must be
responsible for dissipating the wave.Comment: LaTeX 13 pages, 3 figures, typos remove
The origin of peculiar jet-torus structure in the Crab nebula
Recent discoveries of the intriguing ``jet-torus'' structure in the Crab
Nebula and other pulsar nebulae prompted calls for re-examining of their
theory. The most radical proposals involve abolishing of the MHD approximation
altogether and developing of purely electromagnetic models. However, the
classical MHD models of the Crab Nebula were hampered by the assumption of
spherical symmetry made in order to render the flow equations easily
integrable. The impressive progress in computational relativistic
magnetohydrodynamics in recent years has made it possible to study the Crab
nebula via numerical simulations without making such a drastic simplification
of the problem. In this letter we present the results of the first study of
such kind. They show that the jet-torus pattern can be explained within MHD
approximation when anisotropy of pulsar winds is taken into account. They also
indicate that the flow in the nebula is likely to be much more intricate than
it has been widely believed.Comment: Rejected by Nature, submitted to MNRAS Letter
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