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B. The Pulsar Model

Several "inner engine" models involve pulsar like activity of the inner engine which is directly connected to a Poynting flux decimated relativistic flow (in a contrast to a baryonic flux dominated flow). Energy considerations require an extremely large magnetic fields of the order of 1015 G within such sources.

Usov [412] suggested that GRB arise during the formation of rapidly rotating highly magnetized neutron stars. Such objects could form by the gravitational collapse of accreting white dwarfs with anomalously high magnetic fields in binaries, as in magnetic cataclysmic binaries. The rapidly rotating and strongly magnetized neutron stars would lose their rotational kinetic energy on a timescale of seconds or less In a pulsar like mechanism. The energy available in this case is the rotational and magnetic energies of the neutron star that are of the order of a few × 1051 ergs for a neutron star rotating near breakup. The rotation of the magnetic field creates a strong electric field and an electron-positron plasma which is initially optically thick and in quasi-thermodynamic equilibrium. Additionally a very strong magnetic field forms. The pulsar produces a relativistic Poynting flux dominated flow.

While a Poynting flux dominated flow may be dissipated in a regular internal shocks. Usov [413] and Thompson [406] discuss a scheme in which the energy is dissipated from the magnetic field to the plasma and then via plasma instability to the observed gamma-rays outside the gamma-rays photosphere, which is at around 1013 cm. At this distance the MHD approximation of the pulsar wind breaks down and intense electromagnetic waves are generated. The particles are accelerated by these electromagnetic waves to Lorentz factors of 106 and produce the non thermal spectrum. Smolsky and Usov [387, 388] and Drenkhahn and Spruit [85] discuss various aspects of the conversion of the Poynting flux energy to gamma-rays but these issues are more related to the nature of the emitting regions and only indirectly to the nature of the inner engine.

Usov's model is based on rotating highly magnetized neutron star and from this point of view it indeed resembles to a large extend a regular pulsar. Other authors consider pulsar like activities in other contexts. Katz [185], for example, considers a black hole - thick disk model in which the electromagnetic process turn rotational energy to particle energy in a pulsar like mechanism. Mészáros and Rees [237] discuss related idea on the formation of a Poynting flux dominated flow within a black hole accretion disk model.

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