Are GRB jets baryonic/leptonic or magnetic?

摘要

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.