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Prompt GRB emission can carry information about local conditions in a jet and provide a link toward understanding of a GRB progenitor. Conventional interpretation of the prompt emission, regardless of the jet origin, is either synchrotron, perhaps with the (self)‐comptonized component, or thermal. Whereas some GRBs are consistent with this interpretation, many are at odds with it. Particularly, the ubiquitous spectral variability and correlations still awaits its physical interpretation. In view of the recent discovery of the generation of small‐scale magnetic fields during reconnection in pair plasmas and the claimed non‐discovery of synchrotron‐violating spectra in PIC simulations of relativistic shocks the reassessment of physical models of GRB jets and its radiation becomes imperative. Here we argue that the conventional assumption of the isotropy (in the co‐moving frame) of the produced radiation pattern is too simplistic to explain the wealth of observational data. We show that the anisotropic spectra fit observations much better. The anisotropy of radiation can be due to the ambient (progenitor) magnetic field, it can also be induced by the shock dynamics and geometry, or reflect the anisotropy of the post‐shock turbulence. Interestingly, different emission mechanisms (e.g., synchrotron, small‐pitch‐angle, jitter, thermal) can dominate in different environments (e.g., magnetic, baryonic or leptonic jets) and result in different spectro‐temporal characteristics. We show how the corresponding spectral correlations can distinguish between the jet origin and composition or as least rule our certain models.

We review the models of Gamma‐Ray Bursts (GRBs) with relativistic baryon‐dominated ejecta: the Weibel‐jitter model and the vorticity model. The former may also be relevant to the magnetic‐dominated outflows. We discuss the origin of magnetic fields, mechanisms of electron heating/acceleration, nature and properties of the emitted radiation and the relevance of computer modeling to realistic GRBs. We discuss observational effects and predictions that, hopefully, will allow us to discriminate between the models.

Radiation from GRBs, either in the prompt phase or seen as flares or an afterglow, is believed to be produced at shocks by accelerated electrons in magnetic fields. However, there are alternatives, at least for the prompt and flares, namely the magnetically dominated and Poynting flux driven outflows, where no shocks presumably form at all. The outflow properties are intimately linked to the central engine/progenitor; hence, to understand the nature of the outflow is of great importance. Here we review the present status of the “standard model of GRBs”—the model with internal/external shocks. We briefly discuss two alternative models of the magnetic field generation at shocks: the model based on the Weibel instability and the recently proposed one based on the generation of vorticity at a shock. We also address the electron energization/acceleration mechanisms (including non‐Fermi) in both models. The observational predictions for the spectral and temporal dynamics of the produced radiation have been elab...

The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We suggest that these fields could have been produced via the Weibel instability operating at shocks during the Large-Scale Structure formation and at accretion shocks in Galaxy Clusters. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma (with the ion-to-electron mass ratio of 100), in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from tens of nano-Gauss in the intercluster medium to a few micro-Gauss inside galaxy clusters. We discuss whether and how our results may change with the change the mass ratio to the realistic value of 1836. We stress that even if the Weibel-generated small-scale magnetic fields decay with time, they can serve as seed fields that can be further amplified and inverse-cascade to larger scales by turbulent motions of post-shock MHD turbulence.

X‐rays are produced throughout the heliosphere as a consequence of charge transfer collisions between heavy solar wind ions and neutral atoms. After such a collision the solar wind ion is left in a highly excited state and emits extreme ultraviolet and soft X‐ray photons. In the outer heliosphere, solar wind charge exchange X‐ray emission is mainly due to charge exchange with neutral interstellar hydrogen. We have combined MHD simulations with a comprehensive charge exchange computation code. We trace the full evolution of solar wind ions along stream line in order to produce three‐dimensional emissivities and, subsequently, two‐dimensional X‐ray brightness and spectral maps of the heliosphere as would be observed from the outside. The model treats both the collisionally thin and the collisionally thick regimes. This model can be a diagnostic tool for studying stellar wind properties of nearby Sun‐like stars.

We present a theory of ultrarelativistic collisionless shocks based on the relativistic kinetic two-stream instability. We demonstrate that the shock front is unstable to the generation of small-scale, randomly tangled magnetic fields. These fields are strong enough to scatter the energetic incoming (in the shock frame) protons and electrons over pitch angle and, therefore, to convert their kinetic energy of bulk motion into heat with very high efficiency. This validates the use of MHD approximation and the shock jump conditions in particular. The effective collisions are also necessary for the diffusive Fermi acceleration of electrons to operate and produce an observed power-law. Finally, these strong (sub-equipartition) magnetic fields are also required for the efficient synchrotron-type radiation emission from the shocks. The predicted magnetic fields have an impact on polarization properties of the observed radiation (e.g., a linear polarization from a jet-like ejecta and polarization scintillations in radio for a spherical one) and on its spectrum. We present an analytical theory of jitter radiation, which is emitted when the magnetic field is correlated on scales smaller then the gyration (Larmor) radius of the accelerated electrons. A composite jitter+synchrotron model of GRB $\gamma$-ray emission from internal shocks is capable of resolving many puzzles of GRB spectra, such as the violation of the ``line of death'', sharp spectral breaks, and multiple spectral components seen in some bursts (good examples are GRB910503, GRB910402, etc.). We stress that simultaneous detection of both spectral components opens a way to a precise diagnostics of the conditions in GRB shocks. We also discuss the relation of our results to other systems, such as internal shocks in blazars, radio lobes, and supernova shocks., Comment: 5 pages, 4 figures. To appear in proceedings of the Gamma 2001 symposium