Gamma-Ray Bursts as Sources of Strong Magnetic Fields

Granot, Jonathan; Piran, Tsvi; Bromberg, Omer; Racusin, J. L.; Daigne, F.

doi:10.1007/978-1-4939-3550-5_14

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“…They can also affect each other, as the outflow composition affects its dynamics and dissipation, which in turn affect the resulting emission. In particular, a key question is whether the energy is carried out from the central source to the emission region predominantly as kinetic energy-a baryonic jet (Goodman 1986;Paczýnski 1986;Shemi & Piran 1990), or as Poynting flux-a highly magnetized (or Poynting-flux dominated) jet (Usov 1992;Thompson 1994;Mészáros & Rees 1997;Blandford 2002;Lyutikov 2006;Granot et al 2015) with a large magnetization parameter σ (the magnetic-to-particle enthalpy density or energy flux ratio). A baryonic, kinetically dominated jet can naturally lead to reasonably efficient energy dissipation via internal shocks within the outflow (Rees & Mészáros 1994).…”

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confidence: 99%

Gamma-Ray Bursts From Magnetic Reconnection: Variability and Robustness of Light Curves

Granot

2016

ApJL

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The dissipation mechanism that powers gamma-ray bursts (GRBs) remains uncertain almost half a century after their discovery. The two main competing mechanisms are the extensively studied internal shocks and the less studied magnetic reconnection. Here we consider GRB emission from magnetic reconnection accounting for the relativistic bulk motions that it produces in the jetʼs bulk rest frame. Far from the source the magnetic field is almost exactly normal to the radial direction, suggesting locally quasi-spherical thin reconnection layers between regions of oppositely directed magnetic field. We show that if the relativistic motions in the jetʼs frame are confined to such a quasi-spherical uniform layer, then the resulting GRB light curves are independent of their direction distribution within this layer. This renders previous results for a delta-function velocity-direction distribution applicable to a much more general class of reconnection models, which are suggested by numerical simulations. Such models that vary in their velocity-direction distribution differ mainly in the size of the bright region that contributes most of the observed flux at a given emission radius or observed time. The more sharply peaked this distribution, the smaller this bright region, and the stronger the light curve variability that may be induced by deviations from a uniform emission over the thin reconnection layer, which may be expected in a realistic GRB outflow. This is reflected both in the observed image at a given observed time and in the observer-frame emissivity map at a given emission radius, which are calculated here for three simple velocity-direction distributions.

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