Physics Department

Astrophysics Seminar
Why is the GRB prompt emission like an A-bomb?
Demos Kazanas
21/05/2017, 14:30 - 15:30
Lidow, Nathan Rosen (300)


We present a model for the prompt emission of GRB. This model is based on a radiative instability of a relativistic proton plasma, such as that present in the GRB forward shock. The model depends on the pγ --> e+e- reaction to remove energy out of the GRB protons and into e+e- pairs. The pairs in turn produce more photons (via synchrotron) to enhance this reaction rate and so forth, similar to the chain reaction of an A-bomb. Like in an A-bomb, this process has a critical column for the GRB forward shock relativistic protons (instead of that of the U atoms); so it will not go off until enough matter has been swept by the blast wave. Besides that, it also has a kinematic threshold, in that the e+e- synchrotron photons should be able to pair produce with the relativistic protons. It is shown that this threshold, when shifted to the Earth frame, translates into emission at E ~ m_e c^2, to produce the GRB spectrum Band component. The model, thus, resolves two issues of GRB physics: taking energy out of the protons and producing E_peak at near MeV energies. We also show that interactions of the GRB photons with its own , forward scattered, photons can reduce the value of its Lorentz factor by 30%-50% and continues its evolution at a constant (albeit reduced) Lorentz factor. This suffices to stop the transfer of proton energy to electrons, thereby terminating the prompt GRB phase, with the GRB suffering a steep decline in its luminosity, henceforth powered only by the electrons swept by the blast wave. This produces a plateau in its X-ray flux similar to those observed in many GRB afterglows. The statistics of the prompt/plateau luminosities have a distribution with a peak at a value m_p/m_e, as suggested by the model.

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