The Merger of a Helium Star and a Black Hole: Gamma-Ray Bursts

Abstract

There is growing observational evidence that gamma-ray bursts (GRBs) are powered by black holes accreting rapidly through a disk. The supernova-like outburst that accompanies some gamma-ray bursts suggest that some long-duration GRBs may be driven by the accretion of a rotating stellar core onto a central black hole. Such a system can be produced when a compact remnant spirals into the helium core of its binary companion. During the inspiral, orbital angular momentum is injected into the core. By the time the compact remnant reaches the center of the helium core, it too has gained angular momentum as well as mass, producing a rapidly accreting black hole (or neutron star) at the center of a rotating stellar core. In this paper, we use a 3-dimensional smooth particle hydrodynamics (SPH) code to follow such a merger process and make quantitative estimates of the initial mass and spin of the central compact remnant, as well as the angular momentum in the accreting helium core. From these results, we estimate GRB explosion energies. In all mergers, magnetically driven jets are expected to produce GRB explosions with energies above 10^{51} ergs. For neutrino-annihilation-driven explosions, the GRB energy increases dramatically with helium star mass: the merger of a 2 M_sun compact remnant with a 4 M_sun helium star only produces a 10^{47} ergs explosion in \sim 500 s whereas the merger of a 2 M_sun compact remnant with a 16 M_sun helium star produces a >10^{52} ergs explosion in \sim 65 s. (Abridged)