The electromagnetic model of short GRBs, the nature of prompt tails, supernova-less long GRBs and highly efficient episodic accretion

Abstract

Many short GRBs show prompt tails lasting up to hundreds of seconds that can be energetically dominant over the initial sub-second spike. In this paper we develop an electromagnetic model of short GRBs that explains the two stages of the energy release, the prompt spike and the prompt tail. The key ingredient of the model is the recent discovery that an isolated black hole can keep its open magnetic flux for times much longer than the collapse time and, thus, can spin-down electromagnetically, driving the relativistic wind. First, the merger is preceded by an electromagnetic precursor wind. If a fraction of the wind power is converted into pulsar-like coherent radio emission, this may produce an observable radio burst of few milliseconds. At the active stage of the merger, two neutron stars produces a black hole surrounded by an accretion torus in which the amplified magnetic field extracts the rotational energy of the black hole and drives an axially-collimated electromagnetic wind. For observers nearly aligned with the orbital normal this is seen as a classical short GRB. After the accretion of the torus, the isolated black hole keeps the open magnetic flux and drives the equatorially (not axially) collimated outflow, which is seen by an observer at intermediate polar angles as a prompt tail. The tail carries more energy than the prompt spike, but its emission is de-boosted for observers along the orbital normal. Observers in the equatorial plane miss the prompt spike and interpret the prompt tail as an energetic supernova-less long GRB. We also demonstrate that episodic accretion onto the BH of magnetized clouds that carry non-zero magnetic flux can be highly efficient in extracting the spin energy of the BH, producing the outflows with the power exceeding the average accretion power.