We show that the relativistic two-stream instability can naturally generate strong magnetic fields with 10-5-10-1 of the equipartition energy density, in the collisionless shocks of gamma-ray burst (GRB) sources. The generated fields are parallel to the shock front and fluctuate on the very short scale of the plasma skin depth. The synchrotron radiation emitted from the limb-brightened source image is linearly polarized in the radial direction relative to the source center. Although the net polarization vanishes under circular symmetry, GRB sources should exhibit polarization scintillations as their radio afterglow radiation gets scattered by the Galactic interstellar medium. Detection of polarization scintillations could therefore test the above mechanism for magnetic field generation.

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Generation of Magnetic Fields in the Relativistic Shock of Gamma-Ray Burst Sources

We present measurements of the near-field heat transfer between the tip of a thermal profiler and planar material surfaces under ultrahigh vacuum conditions. For tip-sample distances below 10-8 m, our results differ markedly from the prediction of fluctuating electrodynamics. We argue that these differences are due to the existence of a material-dependent small length scale below which the macroscopic description of the dielectric properties fails, and discuss a heuristic model which yields fair agreement with the available data. These results are of importance for the quantitative interpretation of signals obtained by scanning thermal microscopes capable of detecting local temperature variations on surfaces.

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Near-field heat transfer in a scanning thermal microscope.

Gamma-ray burst afterglow polarization is discussed. We find an observable, up to ~10%, polarization, if the magnetic field coherence length grows at about the speed of light after the field is generated at the shock front. Detection of a polarized afterglow would show that collisionless ultrarelativistic shocks can generate strong large-scale magnetic fields and confirm the synchrotron afterglow model. Nondetection, at the ~1% level, would imply that either the synchrotron emission model is incorrect or that strong magnetic fields, after they are generated in the shock, somehow manage to stay undissipated at "microscopic," skin depth, scales. Analytic light curves of synchrotron emission from an ultrarelativistic self-similar blast wave are obtained for an arbitrary electron distribution function, taking into account the effects of synchrotron cooling. The peak synchrotron flux and the flux at frequencies much smaller than the peak frequency are insensitive to the details of the electron distribution function; hence, their observational determination would provide strong constraints on blast-wave parameters.

http://iopscience.iop.org/article/10.1086/306720/pdf

Gamma-ray burst afterglow: Polarization and analytic light curves

Observations by the {\RHESSI} satellite of large polarization of the prompt $\gamma$-ray emission from the Gamma Ray Burst GRB021206 \citep{coburn03} imply that the magnetic field coherence scale is larger than the size of the visible emitting region, $\sim R/\Gamma$, where $R$ is the radius of the flow, $\Gamma$ is the associated Lorentz factor. 
Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs \citep{lyutikov02}, in which large scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive Stokes parameters of synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse averaged polarization fraction of the emission from a relativistically expanding shell carrying global toroidal magnetic field. For viewing angles larger than $1/\Gamma$ the observed patch of the emitting shell has almost homogeneous magnetic field, producing a large fractional polarization ($56% $ for a power-law energy distribution of relativistic particles $dn/d\epsilon \propto \epsilon^{-3}$). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in uniform magnetic field due to relativistic kinematic effects.

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Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow

We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron-positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical, and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factor {Lambda}. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. Our computations predict and explain the following behavior. (1) For jets unresolved in the direction perpendicular to their direction of propagation, the position angle of the electric vector of the linear polarization has a bimodal distribution, being oriented either parallel or perpendicular to the jet. (2) If an ultra-relativistic jet with {Lambda} >> 1 whose axis makes a small angle to the line of sight, {theta} {approx} 1/{Lambda}, experiences a relatively small change in the direction of propagation, velocity or pitch angle of the magnetic fields, the polarization is likely to remain parallel or perpendicular; on the other hand, in some cases, the degree of polarization can exhibit large variations and the polarization position angle can experience abrupt 90{sup o} changes. This change is more likely to occur in jets with flatter spectra. (3)more » In order for the jet polarization to be oriented along the jet axis, the intrinsic toroidal magnetic field (in the frame of the jet) should be of the order of or stronger than the intrinsic poloidal field; in this case, the highly relativistic motion of the jet implies that, in the observer's frame, the jet is strongly dominated by the toroidal magnetic field B{sub {phi}}/B{sub z} {ge} {Lambda}. (4) The emission-weighted average pitch angle of the intrinsic helical field in the jet must not be too small to produce polarization along the jet axis. In force-free jets with a smooth distribution of emissivities, the emission should be generated in a limited range of radii not too close to the jet core. (5) For mildly relativistic jets, when a counter jet can be seen, the polarization of the counter jet is preferentially orthogonal to the axis, unless the jet is strongly dominated by the toroidal magnetic field in its rest frame. (6) For resolved jets, the polarization pattern is not symmetric with respect to jet axis. Under certain conditions, this can be used to deduce the direction of the spin of the central object (black hole or disk), whether it is aligned or anti-aligned with the jet axis. (7) In resolved ''cylindrical shell'' type jets, the central parts of the jet are polarized along the axis, while the outer parts are polarized orthogonal to it, in accordance with observations. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. Since the typical degrees of polarization are {le} 15%, the emitting parts of the jets must have comparable rest-frame toroidal and poloidal fields. In this case, most relativistic jets are strongly dominated by the toroidal magnetic field component in the observer's frame, B{sub {phi}}/B{sub z} {approx} {Lambda}. We also discuss the possibility that relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In this case, dissipation of the toroidal magnetic field (and not fluid shocks) may be responsible for particle acceleration.« less

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Polarization and structure of relativistic parsec-scale AGN jets

The Hamiltonian approach to electrodynamics radiation reaction uniformly accelerated charges radiation emitted by relativistic and non-relativistic moving particles synchrotron radiation electrodynamics of a continuous medium the Cerenkov and Doppler effects transition radiation and transition scattering superluminal sources of radiation reabsorption and transfer of radiation electrodynamics of media with spatial dispersion permittivity and wave propagation in plasmas the energy-momentum tensor and forces in macroscopic electrodynamics, energy and heat liberated in a dispersive absorbing medium fluctuations and van der Waals forces wave scattering in a medium astrophysics of cosmic rays x-ray astronomy gamma-ray astronomy.

V. L. Ginzburg

Papers

Applications of Electrodynamics in Theoretical Physics and Astrophysics