MG15 - Talk detail

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This talk will focus on what can be learned from this event about the properties of the outflow that powered the prompt gamma-ray emission and the afterglow emission, and briefly outline the constraints on the type of compact remnant (black hole or massive neutron star) that was left in its aftermath. Both its prompt gamma-ray emission properties (highly under-luminous but with a more typical peak energy) and the long lasting (more than 130 days) rise of its (X-ray to radio) afterglow flux strongly challenge the simple "top-hat" jet model -- a uniform jet with sharp edges. These observations, and in particular the afterglow, may instead be explained by either of two competing effects: (i) angular energy distribution: an off-axis structured jet, and (ii) radial velocity distribution (in a quasi-spherical outflow). Both types of models can reproduce the observed afterglow flux up to the peak at around 150 days, but differ in their post-peak predictions. In fact, the most recent observations may already start to discriminate between them. Finally, three additional and potentially powerful diagnostics will be discussed: the afterglow polarization, flux centroid proper motion, and image size and shape. These can further help to break the degeneracy between different models.

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Abstract

The prompt gamma-ray emission in gamma-ray bursts (GRBs) sometimes shows a high-energy spectral cutoff. If such a cutoff is indeed caused by intrinsic opacity to pair production then it can be extremely useful in determining the outflow's bulk Lorentz factor (LF). Simple one-zone pair-production opacity estimates have led to LFs between 50 and 800 for a handful of GRBs that show spectral cutoffs at observed energies above 10 MeV. More sophisticated models predict the LF to be lower by a factor of 2. However, all such models ignore the effect of electron-positron pair cascades that can enhance electron cooling and thereby shift the spectral cutoff to a lower energy. This effect becomes very important in high compactness emission regions, where the Thomson optical depth of the produced pairs is much larger than unity. In this talk, I will argue that when the Thomson optical depth of pairs is high then simple one-zone estimates can under-predict the LF by as much as an order of magnitude. I will show spectral fits to the time-resolved spectra of GRBs 100724B and 160509A from a detailed model that self-consistently accounts for the effect of pair cascades.

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