Relativistic Jets from Collapsars

Abstract

We have studied the relativistic beamed outflow proposed to occur in the collapsar model of gamma-ray bursts. A jet forms as a consequence of an assumed energy deposition of ∼ 10 50 − 10 51 erg/s within a 30 • cone around the rotation axis of the progenitor star. The generated jet flow is strongly beamed ( < ∼ few degrees) and reaches the surface of the stellar progenitor (r ≈ 3 10 10 cm) intact. At break-out the maximum Lorentz factor of the jet flow is about 33. Simulations have been performed with the GENESIS multi-dimensional relativistic hydrodynamic code. Motivation and numerical setupVarious catastrophic collapse events have been proposed to explain the energies released in a gamma-ray burst (GRB) including compact binary system mergers [6,13], collapsars [18] and hypernovae [14]. These models all rely on a common engine, namely a stellar mass black hole (BH) which accretes several solar masses of matter from a disk (formed during a merger or by a non-spherical collapse). A fraction of the gravitational binding energy released by accretion is converted into a pair fireball. Provided the baryon load of the fireball is not too large, the baryons are accelerated together with the e + e − pairs to ultra-relativistic speeds (Lorentz factors > 10 2 ; [3]). The existence of such relativistic flows is supported by radio observations of GRB 980425 [8].The dynamics of spherically symmetric relativistic fireballs has been studied by several authors by means of 1D Lagrangian hydrodynamic simulations (e.g., [12]). It has been argued that the rapid temporal decay of several GRB afterglows is more consistent with the evolution of a relativistic jet after it slows down and spreads laterally than with a spherical blast wave [9]. The lack of a significant radio afterglow in GRB 990123 provides independent evidence for jet-like geometry [10]. Motivated by these observations and by the collapsar model of [11], we have simulated the propagation of jets from collapsars using relativistic hydrodynamics.In [11] the continued evolution of rotating helium stars, whose iron core collapse does not produce a successful outgoing shock but instead forms a BH surrounded 1

Compton-dragged Gamma-Ray Bursts Associated with Supernovae

“…a broad black-body continuum peaked at a temperature T drag ∼ 2 Γ 2 T . While the observed characteristic photon energy would be therefore ǫ ∼ 0.5Γ 2 2 T 5 (1 + z) −1 MeV, in good agreement with the observed distribution of peak energies of BATSE GRBs (assuming again Γ = 100, see below), the spectrum would not reproduce the observed smoothly broken power-law shape (Band et al 1993).…”