We consider the long-term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts. The initial encounter impulsively creates a hot, dense, ...neutrino-cooled disk capable of powering the prompt emission. After a long delay, we find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into alpha -particles. The associated energy release depletes the mass supply and eventually shuts off activity of the central engine. As a result, the luminosity and mass accretion rate deviate from the earlier self-similar behavior expected for an isolated ring with efficient cooling. This then enables a secondary episode of delayed activity to become prominent as an observable signature, when material in the tidal tails produced by the initial encounter returns to the vicinity of the central object. The timescale of the new accretion event can reach tens of seconds to minutes, depending on the details of the system. The associated energies and timescales are consistent with those occurring in X-ray flares.
The rotation rate in pre-supernova cores is an important ingredient that can profoundly affect the post-collapse evolution and associated energy release in supernovae and long gamma ray bursts ...(LGRBs). Previous work has focused on whether the specific angular momentum is above or below the critical value required for the creation of a centrifugally supported disk around a black hole. Here, we explore the effect of the distribution of angular momentum with radius in the star and show that qualitative transitions between high and low angular momentum flow, corresponding to high and low luminosity accretion states, can effectively be reflected in the energy output, leading to variability and the possibility of quiescent times in LGRBs.
The dynamics of gamma-ray burst (GRB) jets during the afterglow phase is most reliably and accurately modeled using hydrodynamic simulations. All published simulations so far, however, have ...considered only a uniform external medium, while a stratified external medium is expected around long duration GRB progenitors. Common to all calculations is the initiation of the GRB jet as a conical wedge of half-opening angle theta sub(0) = 0.2 whose radial profile is taken from the self-similar Blandford-McKee solution. The dynamics for stratified external media (k = 1, 2) are broadly similar to those derived for expansion into a uniform external medium (k = 0). The jet half-opening angle is observed to start increasing logarithmically with time (or radius) once the Lorentz factor Gamma drops below theta super(-1) sub(0). The counterjet becomes visible as it becomes sub-relativistic, and for k = 0 this results in a clear bump-like feature in the light curve.
We consider accretion onto newborn black holes following the collapse of rotating massive stellar cores, at the threshold where a centrifugally supported disk gives way to nearly radial inflow for ...low angular momentum. For realistic initial conditions taken from pre-supernova (pre-SN) evolution calculations, the densities and temperatures involved require the use of a detailed equation of state and neutrino cooling processes, as well as a qualitative consideration of the effects of general relativity. Through two-dimensional dynamical calculations we show how the energy release is affected by the rotation rate and the strength of angular momentum transport, giving rise to qualitatively different solutions in limits of high and low angular momentum, each being capable of powering a gamma-ray burst (GRB). We explore the likelihood of producing Fe-group elements in the two regimes and suggest that while large and massive centrifugally supported disks are capable of driving strong outflows with a possible SN-like signature, quasi-radial flows lack such a feature and may produce a GRB without such an accompanying feature, as seen in GRB060505.
We report on the development of Mezcal-SRHD, a new adaptive mesh refinement, special relativistic hydrodynamics (SRHD) code, developed with the aim of studying the highly relativistic flows in ...gamma-ray burst sources. The SRHD equations are solved using finite-volume conservative solvers, with second-order interpolation in space and time. The correct implementation of the algorithms is verified by one-dimensional (1D) and multi-dimensional tests. The code is then applied to study the propagation of 1D spherical impulsive blast waves expanding in a stratified medium with rho is proportional to r super(-k), bridging between the relativistic and Newtonian phases (which are described by the Blandford-McKee and Sedov-Taylor self-similar solutions, respectively), as well as to a two-dimensional (2D) cylindrically symmetric impulsive jet propagating in a constant density medium. It is shown that the deceleration to nonrelativistic speeds in one dimension occurs on scales significantly larger than the Sedov length. This transition is further delayed with respect to the Sedov length as the degree of stratification of the ambient medium is increased. This result, together with the scaling of position, Lorentz factor, and the shock velocity as a function of time and shock radius, is explained here using a simple analytical model based on energy conservation. The method used for calculating the afterglow radiation by post-processing the results of the simulations is described in detail. The light curves computed using the results of 1D numerical simulations during the relativistic stage correctly reproduce those calculated assuming the self-similar Blandford-McKee solution for the evolution of the flow. The jet dynamics from our 2D simulations and the resulting afterglow light curves, including the jet break, are in good agreement with those presented in previous works. Finally, we show how the details of the dynamics critically depend on properly resolving the structure of the relativistic flow.
We consider the long-term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts. The initial encounter impulsively creates a hot, dense, ...neutrino-cooled disk capable of powering the prompt emission. After a long delay, we find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into {alpha}-particles. The associated energy release depletes the mass supply and eventually shuts off activity of the central engine. As a result, the luminosity and mass accretion rate deviate from the earlier self-similar behavior expected for an isolated ring with efficient cooling. This then enables a secondary episode of delayed activity to become prominent as an observable signature, when material in the tidal tails produced by the initial encounter returns to the vicinity of the central object. The timescale of the new accretion event can reach tens of seconds to minutes, depending on the details of the system. The associated energies and timescales are consistent with those occurring in X-ray flares.
The dynamics of GRB jets during the afterglow phase is most reliably and accurately modelled using hydrodynamic simulations. All published simulations, however, have considered only a uniform ...external medium, while a stratified external medium is expected around long duration GRB progenitors. Here we present simulations of the dynamics of GRB jets and the resulting afterglow emission for both uniform and stratified external media with \(\rho \propto r^{-k}\) for k = 0, 1, 2. The simulations are performed in 2D using the special relativistic version of the Mezcal code. The dynamics for stratified external media are broadly similar to those derived for expansion into a uniform external medium. The jet half-opening angle start increasing logarithmically with time once the Lorentz factor drops below 1/theta_0. For larger k values the lateral expansion is faster at early times and slower at late times with the jet expansion becoming Newtonian and slowly approaching spherical symmetry over progressively longer timescales. We find that contrary to analytic expectations, there is a reasonably sharp jet break in the lightcurve for k = 2 although the shape of the break is affected more by the viewing angle than by the slope of the external density profile. Steeper density profiles are found to produce more gradual jet breaks while larger viewing angles cause smoother and later appearing jet breaks. The counter-jet becomes visible as it becomes sub-relativistic, and for k=0 this results in a clear bump-like feature in the light curve. However, for larger k values the jet decelerates more gradually, causing only a mild flattening in the radio light curve that might be hard to discern when k=2. Late time radio calorimetry is likely to consistently over-estimate the true energy by up to a factor of a few for k=2, and either over-predict or under-predict it by a smaller factor for k = 0,1.
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