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Numerica and Analytic Study of Relativistic Jets

Final Activity Report Summary - RGENESIS (Numerica and analytic study of relativistic jets)

This project aimed to study the variability properties of relativistic outflows, focusing on the implications to the observed light curves of rapidly varying compact sources of non-thermal radiation such as gamma-ray bursts (GRBs) and blazars. To study them we used the internal shock model, (applied to blazars and to the prompt phase of a GRB) and the external shock model (applied to the GRB afterglow phase). Internal shocks are generated by collisions of dense shells in a relativistic outflow whose velocity is non-uniform, while the interaction between the shell and the interstellar medium generates external shocks in the later stages of a GRB. Relativistic shocks accelerate particles to very high energies, and the radiation which is observed on Earth is caused by the synchrotron radiation emitted by these particles emit gyrating in the magnetic field behind the shocks. We computed the evolution and synthetic observations using a sophisticated relativistic magnetohydrodynamic code MRGENESIS which enabled us to perform high resolution simulations of magnetised shell evolution and collisions under extreme conditions generated by the relativistic outflows of GRBs and blazars (velocities very close to the speed of light, high density and pressure contrasts, highly magnetised outflows).

We find that magnetised fluid has distinct observable properties from the non-magnetised one. In the case of the internal shocks in blazars we computed the synthetic observable light curves and saw that those originating in the magnetised flow have a more complex and richer morphology than those from the non-magnetised flow. Many of the blazar flares observed by X-ray satellites sometimes have such a form, so that the presence of strong magnetic fields in blazar jets cannot be ruled out.

When studying interaction of strongly magnetised jet with the surrounding medium, as can be found in the case of the GRB afterglows, we found out that when an appreciable fraction of the flow energy is stored in the magnetic field the initial formation of (reverse) shocks in the jet can be suppressed. However, we discovered that, due to the expansion of the fluid, a delayed formation of the reverse shock is possible, but this shock is expected to be weak. Thus the so-called optical flash predicted for some of the non-magnetised afterglows and associated to the reverse shock is probably suppressed as well. We believe and our work strongly points towards the fact that magnetic field can be responsible for the paucity of observed optical flashes.

On the technical side, we have devised a method which overcomes the limitations of present-day numerical codes when dealing with the velocities extremely close to the speed of light. This method and the parallel computation on supercomputers enables us to properly capture physics of blazars and GRBs, follow the evolution for a sufficiently long time and run a large enough number of models in order to be able to improve our understanding of the connection between the observed radiation and the not directly observable magnetohydrodynamic properties of relativistic outflows.