Collimated Jet or Expanding Outflow: Possible Origins of GRBs and X-Ray Flashes

Abstract

We investigate the dynamics of an injected outflow propagating in a progenitor in the context of the collapsar model for gamma-ray bursts (GRBs) through two dimensional axisymmetric relativistic hydrodynamic simulations. Initially, we locally inject an outflow near the center of a progenitor. We calculate 25 models, in total, by fixing its total input energy to be 10^{51} ergs s^{-1} and radius of the injected outflow to be $7\times 10^7$ cm while varying its bulk Lorentz factor, $Γ_{0} = 1.05\sim 5$, and its specific internal energy, $ε_0/c^2 = 0.1\sim 30$. The injected outflow propagates in the progenitor and drives a large-scale outflow or jet. We find a smooth but dramatic transition from a collimated jet to an expanding outflow among calculated models. The maximum Lorentz factor is, on the other hand, sensitive to both of $Γ_0$ and $ε_0$; roughly $Γ_{\rm max} \sim Γ_0 (1+ε_0/c^2)$. Our finding will explain a smooth transition between the GRBs, X-ray rich GRBs (XRRs) and X-ray Flashes (XRFs) by the same model but with different $ε_0$ values.

Comments 51 pages, 21 figures. accepted for publication in ApJ high resolution version is available at http://www.mpa-garching.mpg.de/~mizuta/COLLAPSAR/collapsar.html