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1.
Mikhail V. Medvedev 《Astrophysics and Space Science》2007,307(1-3):245-250
It has recently been realized that the Weibel instability plays a major role in the formation and dynamics of astrophysical
shocks of gamma-ray bursts and supernovae. Thanks to technological advances in the recent years, experimental studies of the
Weibel instability are now possible in laser-plasma interaction devices. We, thus, have a unique opportunity to model and
study astrophysical conditions in laboratory experiments – a key goal of the Laboratory Astrophysics program. Here we briefly
review the theory of strong non-magnetized collisionless GRB and SN shocks, emphasizing the crucial role of the Weibel instability
and discuss the properties of radiation emitted by (isotropic) electrons moving through the Weibel-generated magnetic fields,
which is referred to as the jitter radiation. We demonstrate that the jitter radiation field is anisotropic with respect to
the direction of the Weibel current filaments and that its spectral and polarization characteristics are determined by microphysical
plasma parameters. We stress that the spectral analysis can be used for accurate diagnostics of the plasma conditions in laboratory
experiments and in astrophysical GRB and SN shocks. 相似文献
2.
The properties of the magnetostatic structures in a collisionless relativistic plasma found by exactly solving the nonlinear
self-consistent kinetic and Maxwell equations using the method of invariants of particle motion are analyzed. These structures
include individual neutral current sheets and cylindrically symmetric filaments as well as their ensembles, admitting a wide
variety of types of particle energy distribution functions. Relationships are established between the possible parameters
of the current sheets and filaments—their sizes, currents and magnetic fields, and the degree of anisotropy of the particle
momentum distribution. The extent to which these parameters are conditioned by the properties of the Weibel instability that
can produce these structure is also discussed. The spectral peculiarities of the synchrotron radiation from the particles
that form the current sheets and filaments are investigated in the special case of power-law energy distribution functions. 相似文献
3.
In this concise review of the recent developments in relativistic shock theory in the Universe we restrict ourselves to shocks
that do not exhibit quantum effects. On the other hand, emphasis is given to the formation of shocks under both non-magnetised
and magnetised conditions. We only briefly discuss particle acceleration in relativistic shocks where much of the results
are still preliminary. Analytical theory is rather limited in predicting the real shock structure. Kinetic instability theory
is briefed including its predictions and limitations. A recent self-similar relativistic shock theory is described which predicts
the average long-term shock behaviour to be magnetised and to cause reasonable power-law distributions for energetic particles.
The main focus in this review is on numerical experiments on highly relativistic shocks in (i) pair and (ii) electron-nucleon
plasmas and their limitations. These simulations do not validate all predictions of analytic and self-similar theory and so
far they do not solve the injection problem and the self-modification by self-generated cosmic rays. The main results of the
numerical experiments discussed in this review are: (i) a confirmation of shock evolution in non-magnetised relativistic plasma
in 3D due to either the lepton-Weibel instability (in pair plasmas) or to the ion-Weibel instability; (ii) the sensitive dependence
of shock formation on upstream magnetisation which causes suppression of Weibel modes for large upstream magnetisation ratios
σ>10−3; (iii) the sensitive dependence of particle dynamics on the upstream magnetic inclination angle θ
Bn
, where particles of θ
Bn
>34° cannot escape upstream, leading to the distinction between ‘subluminal’ and ‘superluminal’ shocks; (iv) particles in
ultra-relativistic shocks can hardly overturn the shock and escape to upstream; they may oscillate around the shock ramp for
a long time, so to speak ‘surfing it’ and thereby becoming accelerated by a kind of SDA; (v) these particles form a power-law
tail on the downstream distribution; their limitations are pointed out; (vi) recently developed methods permit the calculation
of the radiation spectra emitted by the downstream high-energy particles; (vii) the Weibel-generated downstream magnetic fields
form large-amplitude vortices which could be advected by the downstream flow to large distances from the shock and possibly
contribute to an extended strong field region; (viii) if cosmic rays are included, Bell-like modes can generate upstream magnetic
turbulence at short and, by diffusive re-coupling, also long wavelengths in nearly parallel magnetic field shocks; (ix) advection
of such large-amplitude waves should cause periodic reformation of the quasi-parallel shock and eject large-amplitude magnetic
field vortices downstream where they contribute to turbulence and to maintaining an extended region of large magnetic fields. 相似文献
4.
M. E. Dieckmann B. Eliasson M. Parviainen P. K. Shukla A. Ynnerman 《Monthly notices of the Royal Astronomical Society》2006,367(3):865-872
Initially, inhomogeneous plasma jets, ejected by active galactic nuclei and associated with gamma-ray bursts, are thermalized by the formation of internal shocks. Jet subpopulations can hereby collide at Lorentz factors of a few. As the resulting relativistic shock expands into the upstream plasma, a significant fraction of the upstream ions is reflected. These ions, together with downstream ions that leak through the shock, form relativistic beams of ions that outrun the shock. The thermalization of these beams via the two-stream instability is thought to contribute significantly to plasma heating and particle acceleration by the shock. Here, the capability of a two-stream instability to generate relativistic field-aligned and cross-field electron flow, is examined for a magnetized plasma by means of a particle-in-cell (PIC) simulation. The electrons interact with the developing quasi-electrostatic waves and oblique magnetic fields. The simulation results bring forward evidence that such waves, by their non-linear interactions with the plasma, produce a highly relativistic field-aligned electron flow and electron energies, which could contribute to the radio synchrotron emissions from astrophysical jets, to ultrarelativistic leptonic subpopulations propagating with the jet and to the halo particles surrounding the accretion disc of the black hole. 相似文献
5.
We present results of analytical studies and 2D3V PIC simulations of electron-positron plasma cloud collisions. We concentrate
on the problem of quasi-static magnetic field generation. It is shown from linear theory, using relativistic two-fluid equations
for electron-positron plasmas, that the generation of a quasi-static magnetic field can be associated with the counter-streaming
instability. A two-dimensional relativistic particle simulation provides good agreement with the above linear theory and that,
in the nonlinear stage of the instability, about 5.3% of the initial plasma flow energy can be converted to magnetic field
energy. It is also shown from the simulation that the quasi-static magnetic field undergoes a collision-less change of structure,
leading to large scale, long living structures and the production of high-energy particles. These processes may be important
for understanding of production of high-energy particles in the region where two pulsar winds collide.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
K. I. Kellermann Y. Y. Kovalev M. L. Lister D. C. Homan M. Kadler M. H. Cohen E. Ros J. A. Zensus R. C. Vermeulen M. F. Aller H. D. Aller 《Astrophysics and Space Science》2007,311(1-3):231-239
We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets
with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the
observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity,
the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed.
Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its
proximity, is observed with excellent spatial resolution.
Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different
features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The
data are best fit with a distribution of Lorentz factors extending up to γ∼30 and intrinsic luminosity up to ∼1026 W Hz−1. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed
brightness temperature near the base of the jet extend up to ∼5×1013 K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic
energy. However, more typically, the observed brightness temperatures are ∼2×1011 K, i.e., closer to equipartition. 相似文献
7.
We study the parametric decays of an electromagnetic wave propagating along an external magnetic field in an electron-positron plasma. We include weakly relativistic effects on the particle motions in the wave field, and the nonlinear ponderomotive force. We find resonant and nonresonant wave couplings. These include, ordinary decay instabilities, in which the pump wave decays into an electro-acoustic mode and a sideband wave. There are also nonresonant couplings involving two sideband waves, and a nonresonant modulational instability in which the pump wave decays into two sideband modes. Depending on the parameters involved, there is a resonant modulational instability involving a forward propagating electro-acoustic mode and a sideband daughter wave. 相似文献
8.
Gamma-ray bursts from internal shocks in a relativistic wind: temporal and spectral properties 总被引：1，自引：0，他引：1
We construct models for gamma-ray bursts in which the emission comes from internal shocks in a relativistic wind with a highly non-uniform distribution of the Lorentz factor. We follow the evolution of the wind using a very simplified approach in which a large number of layers interact by direct collisions but all pressure waves have been suppressed. We suppose that the magnetic field and the electron Lorentz factor reach large equipartition values in the shocks. Synchrotron photons emitted by the relativistic electrons have a typical energy in the gamma-ray range in the observer frame. Synthetic bursts are constructed as the sum of the contributions from all the internal elementary shocks, and their temporal and spectral properties are compared with the observations. We reproduce the diversity of burst profiles, the 'FRED' shape of individual pulses and the short time-scale variability. Synthetic bursts also satisfy the duration–hardness relation and individual pulses are found to be narrower at high energy, in agreement with the observations. These results suggest that internal shocks in a relativistic wind may indeed be at the origin of gamma-ray bursts. A potential problem, however, is the relatively low efficiency of the dissipation process. If the relativistic wind is powered by accretion from a disc to a stellar mass black hole, it implies that a substantial fraction of the available energy is injected into the wind. 相似文献
9.
The maximum magnetic field strength generated by Weibel-type plasma instabilities is estimated for typical conditions in the interstellar medium. The relevant kinetic dispersion relations are evaluated by conducting a parameter study both for Maxwellian and for suprathermal particle distributions showing that micro Gauss magnetic fields can be generated. It is shown that, depending on the streaming velocity and the plasma temperatures, either the longitudinal or a transverse instability will be dominant. In the presence of an ambient magnetic field, the filamentation instability is typically suppressed while the two-stream and the classic Weibel instability are retained. 相似文献
10.
It is shown that the particle inertia can cause a tearing instability in an electron-positron collisionless plasma with sheared magnetic fields. An approximate analytical expression for the growth rate is obtained. It characterizes the magnetic reconnection timescale in a magnetized electronpositron plasma. 相似文献
11.
Collisionless shocks in turbulent space plasmas accelerate particles by the Fermi mechanism to ultrarelativistic energies. The interaction of accelerated particles with the plasma inflow produces extended supersonic MHD flows of multicomponent plasma. We investigate the instabilities of a flow of three-component turbulent plasma with relativistic particles against long-wavelength perturbations with scales larger than the accelerated particle transport mean free path and the initial turbulence scales. The presence of turbulence allows us to formulate the system of single-fluid equations, the equation of motion for the medium as a whole, and the induction equation for the magnetic field with turbulent magnetic and kinematic viscosities. The current of accelerated particles enters into the induction equation with an effective magnetic diffusion coefficient. We have calculated the local growth rates of the perturbations related to the nonresonant long-wavelength instability of the current of accelerated particles for MHD perturbations in the WKB approximation. The amplification of long-wavelength magnetic field perturbations in the flow upstream of the shock front can affect significantly the maximum energies of the particles accelerated by a collisionless shock and can lead to the observed peculiarities of the synchrotron X-ray radiation in supernova remnants. 相似文献
12.
The ultrahigh-energy (>20 TeV ) gamma rays emitted by active galactic nuclei can be absorbed in intergalactic space through the production of electron-positron pairs during their interaction with extragalactic background photon fields. The electrons and positrons produced by this interaction form an electromagnetic halo. We have studied the halo formation and calculated the halo radiation spectrum. The magnetic field in the halo formation region is assumed to be strong enough for the electron velocities to be isotropized. For such fields, the halo formation process can be described by the method of generations. We calculated the synchrotron and Compton backscattering radiation spectra for the total halo luminosity. We obtained the spatial distribution of the radiation for a point gamma-ray source. 相似文献
13.
M. Fiore L. O. Silva C. Ren M. A. Tzoufras W. B. Mori 《Monthly notices of the Royal Astronomical Society》2006,372(4):1851-1855
The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios. 相似文献
14.
Akira Mizuta Tatsuya Yamasaki Shigehiro Nagataki Shin Mineshige 《Astrophysics and Space Science》2007,307(1-3):23-27
We investigate the outflow propagation in the collapsar in the context of gamma-ray bursts (GRBs) with 2D relativistic hydrodynamic
simulations. We vary the specific internal energy and bulk Lorentz factor of the injected outflow from non-relativistic regime
to relativistic one, fixing the power of the outflow to be 1051erg s−1. We observed the collimated outflow, when the Lorentz factor of the injected outflow is roughly greater than 2. To the contrary,
when the velocity of the injected outflow is slower, the expanding outflow is observed. The transition from collimated jet
to expanding outflow continuously occurs by decreasing the injected velocity. Different features of the dynamics of the outflows
would cause the difference between the GRBs and similar phenomena, such as, X-ray flashes. 相似文献
15.
We show that the excellent optical and gamma-ray data available for GRB 080319B rule out the internal shock model for the prompt emission. The data instead point to a model in which the observed radiation was produced close to the deceleration radius (∼1017 cm) by a turbulent source with random Lorentz factors of ∼10 in the comoving frame. The optical radiation was produced by synchrotron emission from relativistic electrons, and the gamma-rays by inverse-Compton scattering of the synchrotron photons. The gamma-ray emission originated both in eddies and in an inter-eddy medium, whereas the optical radiation was mostly from the latter. Therefore, the gamma-ray emission was highly variable whereas the optical was much less variable. The model explains all the observed features in the prompt optical and gamma-ray data of GRB 080319B. We are unable to determine with confidence whether the energy of the explosion was carried outwards primarily by particles (kinetic energy) or magnetic fields. Consequently, we cannot tell whether the turbulent medium was located in the reverse shock (we can rule out the forward shock) or in a Poynting-dominated jet. 相似文献
16.
The nonlinear coupling between electromagnetic fields in a strongly magnetized electron-positron plasma is considered. We point out that compressional magnetic field perturbations are excited by the rotational part of the nonlinear current, and derive a new nonlinear system of equations that is basic for studies of modulational instabilities and coherent nonlinear structures in magnetized electron-positron plasmas. 相似文献
17.
The acceleration mechanisms of relativistic jets are of great importance for understanding various astrophysical phenomena such as gamma-ray bursts,active galactic nuclei and microquasars.One of the most popular scenarios is that the jets are initially Poynting-flux dominated and succumb to magnetohydrodynamic instability leading to magnetic reconnections.We suggest that the reconnection timescale and efficiency could strongly depend on the geometry of the jet,which determines the length scale on which the orientations of the field lines change.In contrast to a usuallyassumed conical jet,the acceleration of a collimated jet can be found to be more rapid and efficient(i.e.a much more highly saturated Lorentz factor can be reached)while the jets with lateral expansion show the opposite behavior.The shape of the jet could be formed due to the lateral squeezing on the jet by the stellar envelope of a collapsing massive star or the interaction of the jet with stellar winds. 相似文献
18.
P. K. Shukla 《Astrophysics and Space Science》1985,114(2):381-385
A nonlinear Schrödinger equation is obtained for linearly polarized electromagnetic waves propagating across the ambient magnetic field in an electron-positron plasma. The nonlinearities arising from wave intensity induced particle mass modulation, as well as harmonic generation are incorporated. Modulational instability and localization of pulsar radiation are investigated. 相似文献
19.
20.
Evgeny V. Derishev 《Astrophysics and Space Science》2007,309(1-4):157-161
We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through
synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime
can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings
enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameter-free fast cooling
regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active
galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is
negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration,
works in relativistic shocks. 相似文献