Relativistic jets from collapsars

Using a collapsar progenitor model of MacFadyen & Woosley, we have simulate d the propagation of an axisymmetric jet through a collapsing rotating mass ive star with the GENESIS multidimensional relativistic hydrodynamic code. The jet forms as a consequence of an assumed (constant or variable) energy deposition in the range of 10(50)-10(51) ergs s(-1) within a 30 degrees con e around the rotation axis. The jet flow is strongly beamed (approximately less than a few degrees), spatially inhomogeneous, and time dependent. The jet reaches the surface of the stellar progenitor (R* = 2.98 x 10(10) cm) i ntact. At breakout, the maximum Lorentz factor of the jet flow is 33. After breakout, the jet accelerates into the circumstellar medium, whose density is assumed to decrease exponentially and then become constant, rho(ext) = 10(-5) g cm(-3). Outside the star, the flow begins to expand laterally also (v similar to c), but the beam remains very well collimated. At a distance of 2.54 R*, where the simulation ends, the Lorentz factor has increased to 44.