AMRVAC and Relativistic Hydrodynamic simulations for GRB afterglow phases

We apply a novel adaptive mesh refinement code, AMRVAC, to numerically investigate the various evolutionary phases in the interaction of a relativistic shell with its surrounding cold Interstellar Medium (ISM). We do this for both 1D isotropic as well as full 2D jetlike fireball models. This is relevant for Gamma Ray Bursts, and we demonstrate that, thanks to the AMR strategy, we resolve the internal structure of the shocked shell-ISM matter, which will leave its imprint on the GRB afterglow. We determine the deceleration from an initial Lorentz factor $\gamma=100$ up to the almost Newtonian $\gamma\sim{\cal O}(2)$ phase of the flow. We present axisymmetric 2D shell evolutions, with the 2D extent characterized by their initial opening angle. In such jetlike GRB models, we discuss the differences with the 1D isotropic GRB equivalents. These are mainly due to thermally induced sideways expansions of both the shocked shell and shocked ISM regions. We found that the propagating 2D ultrarelativistic shell does not accrete all the surrounding medium located within its initial opening angle. Part of this ISM matter gets pushed away laterally and forms a wide bow-shock configuration with swirling flow patterns trailing the thin shell. The resulting shell deceleration is quite different from that found in isotropic GRB models. As long as the lateral shell expansion is merely due to ballistic spreading of the shell, isotropic and 2D models agree perfectly. As thermally induced expansions eventually lead to significantly higher lateral speeds, the 2D shell interacts with comparably more ISM matter and decelerates earlier than its isotropic counterpart.Comment: 12 pages, accepted in MNRAS, 12/01/200

Flares in GRB afterglows from delayed magnetic dissipation

One of the most intriguing discoveries made by the Swift satellite is the flaring activity in about half of the afterglow lightcurves. Flares have been observed on both long and short duration GRBs and on time scales that range from minutes to ~1 day after the prompt emission. The rapid evolution of some flares led to the suggestion that they are caused by late central engine activity. Here, I propose an alternative explanation that does not need reviving of the central engine. Flares can be powered by delayed magnetic dissipation in strongly magnetized (i.e. with initial Poynting to kinetic flux ratio \simmore 1) ejecta during its deceleration due to interaction with the external medium. A closer look at the length scales of the dissipation regions shows that magnetic dissipation can give rise to fast evolving and energetic flares. Multiple flares are also expected in the context of the model.Comment: 5 pages, accepted for publication in A&A Letter

Multiwavelength afterglow light curves from magnetized GRB flows

We use high-resolution relativistic MHD simulations coupled with a radiative transfer code to compute multiwavelength afterglow light curves of magnetized ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The aim of our study is to determine how the magnetization of the ejecta at large distance from the central engine influences the afterglow emission, and to assess whether observations can be reliably used to infer the strength of the magnetic field. We find that, for typical parameters of the ejecta, the emission from the reverse shock peaks for magnetization $\sigma_0 \sim 0.01 - 0.1$ of the flow, and that it is greatly suppressed for higher $\sigma_0$. The emission from the forward shock shows an achromatic break shortly after the end of the burst marking the onset of the self-similar evolution of the blast wave. Fitting the early afterglow of GRB 990123 and 090102 with our numerical models we infer respective magnetizations of $\sigma_0 \sim 0.01$ and $\sigma_0 \sim 0.1$ for these bursts. We argue that the lack of observed reverse shock emission from the majority of the bursts can be understood if \sigma_0 \simmore 0.1, since we obtain that the luminosity of the reverse shock decreases significantly for $\sigma_0 \sim 1$. For ejecta with \sigma_0 \simmore 0.1 our models predict that there is sufficient energy left in the magnetic field, at least during an interval of ~10 times the burst duration, to produce a substantial emission if the magnetic energy can be dissipated (for instance, due to resistive effects) and radiated away.Comment: 9 pages, 9 figures. Submitted to MNRAS