Intermittent hydrodynamic jets in collapsars do not produce GRBs
Strong variability is a common characteristic of the prompt emission of GRBs. This observed variability is widely attributed to an intermittency of the central engine, through formation of strong internal shocks in the GRB-emitting jet expelled by the engine. In this paper we study numerically the propagation of hydrodynamic jets, injected periodically by a variable engine, through the envelope of a collapsed star. By post-processing the output of 3D numerical simulations, we compute the net radiative efficiency of the outflow. We find that all intermittent jets are subject to heavy baryon contamination that inhibits the emission at and above the photosphere well below detection limits. This is in contrast to continuous jets that, as shown recently, produce a highly variable gamma-ray photospheric emission with high efficiency, owing to the interaction of the jet with the stellar envelope. Our results challenge the variable engine model for hydrodynamic jets, and may impose constraints on the duty cycle of GRB engines. If such systems exist in nature, they are not expected to produce bright gamma-ray emission, but should appear as X-ray, optical and radio transients that resemble a delayed GRB afterglow signal. Link to the paper
High efficiency photospheric emission entailed by formation of a collimation shock in gamma-ray bursts
As the jet drills through a dense medium, it generates a cocoon that applies pressure on the jet and collimates it. The collimation can considerably enhance the efficiency of the photospheric emission, depends on the baryon loading. 3D numerical simulations feature a substantial stratification of the outflow as well as sporadic loading, even if the injected jet is uniform and continuous. One consequence of this mixing is a strong angular dependence of the radiative efficiency. Another is large differences in the Lorentz factor of different fluid elements that lead to formation of internal shocks. Our analysis indicates that in both long and short GRBs a prominent photospheric component cannot be avoided when observed within an angle of a few degrees to the axis, unless the asymptotic Lorentz factor is limited by baryon loading at the jet base to a terminal Γ<100. Photon generation by newly created pairs behind the collimation shock regulates the observed temperature at 50/θ keV, where θ is the initial jet opening angle, in remarkable agreement with the observed peak energies of prompt emission spectra. Link to the paper
A semi-analytical calculationof the jet propagation based on numerical simulations (August 2017)
For the visual interface and a csv file for a given set of jet and medium parameters, click here. For the python code, click here. Link to the paper
Cocoon emission from LGRBs
A jet is launched from the core of a four solar mass star. The jet spends about two seconds in the star before it breaks out. Once it breaks out from the star, so does the cocoon, for which we calculate numerically the upcoming cooling emission.