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After a black hole consumes a star, why do gamma ray bursts only shoot out perpendicular to the spinning accretion disk? Why do they not shoot out parallel to the disk?

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  • $\begingroup$ The source of the gamma rays is thought to be within the twisted magnetic fields along the poles of the spinning black hole. The source is also thought to be relativistic particles. Since the radiation pattern from a fast moving source is "warped" due to relativistic effects, the result is a beam-like emission. $\endgroup$ – honeste_vivere Dec 3 '15 at 13:46
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Gamma ray bursts originate from the acceleration of charged particles along the rotation axis of the dying star (at least in theory). The gamma rays themselves are beamed and intensified in the direction of motion of the emitting particles - a process called relativistic beaming.

The jets of charged particles are thought to be powered by the coupling of the rotation of the star to its magnetic field. The twisted field lines produced a strong magnetic gradient that is able to accelerate the particles. The direction of that gradient, and hence the acceleration, is along the rotation axis.

Roughly speaking, the opening angle of the beam is given by $1/\gamma$ (in radians), where $\gamma = (1 - v^2/c^2)^{-1/2}$ is the Lorentz factor of the charged particles moving with speed $v$. These factors can be estimated from GRB afterglows and are found to be in the range of 10-1000 (Hascoet et al. 2013; Tang et al. 2014), leading to opening angles for the beam of 0.06-6 degrees. Thus little or no radiation emerges parallel to the disk.

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  • $\begingroup$ @astrophysicistkev, note that the "beam" described here is that of the observed radiation --- which is somewhat distinct from the opening angle of the "jet", which produces the transient. $\endgroup$ – DilithiumMatrix Dec 8 '15 at 18:56

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