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

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 core of its binary companion. During the inspiral, orbital angular momentum is injected into the helium 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. Whether or not such a merger (termed He-merger) can produce a GRB depends upon the initial mass and spin of the central black hole, as well as the angular momentum in the stellar core. In this paper, we use a 3-dimensional smooth particle hydrodynamics (SPH) code to follow the He-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. During the inspiral, a 2M⊙ compact remnant gains ∼ 0.5 − 3.5M⊙ depending on the mass of the helium core (more massive cores provide more accretion). The accretion rates on the central remnant are initially very high, 10 M⊙ yr −1 up to 10 M⊙ yr −1 (the accretion rate increases with increasing helium star mass), and the central remnant quickly becomes a black hole if it was not one already. From these accretion rates, we estimate GRB explosion energies. In all mergers, magnetically driven jets are expected to produce GRB explosions with energies above 10 ergs. For neutrino-annihilation-driven explosions, the GRB energy increases dramatically with helium star mass: the merger of a 2M⊙ compact remnant with a 4M⊙ helium star only produces a 10 erg explosion in ∼ 500 s whereas the merger of a 2M⊙ compact remnant with a 16M⊙ helium star produces a > 10 52 erg explosion in ∼ 65 s. Subject headings: gamma-rays: bursts – binaries: close – black holes