High efficiency photospheric emission in gamma-ray bursts. (arXiv:1904.07244v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gottlieb_O/0/1/0/all/0/1">Ore Gottlieb</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Levinson_A/0/1/0/all/0/1">Amir Levinson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nakar_E/0/1/0/all/0/1">Ehud Nakar</a>

The primary dissipation mechanism in jets of gamma-ray bursts (GRBs), and the
high efficiency of the prompt emission are long standing issues. One
possibility is strong collimation of a weakly magnetized relativistic jet by
the surrounding medium, which can considerably enhance the efficiency of the
photospheric emission. We derive a simple analytic criterion for the radiative
efficiency of a collimated jet showing that it depends most strongly on the
baryon loading. We confirm this analytic result by 3D numerical simulations,
and further find that mixing of jet and cocoon material at the collimation
throat leads to a substantial stratification of the outflow as well as sporadic
loading, even if the injected jet is uniform and continuous. One consequence of
this mixing is a strong angular dependence of the radiative efficiency. Another
is large differences in the Lorentz factor of different fluid elements that
lead to formation of internal shocks. Our analysis indicates that in both long
and short GRBs a prominent photospheric component cannot be avoided when
observed within an angle of a few degrees to the axis, unless the asymptotic
Lorentz factor is limited by baryon loading at the jet base to $Gamma_infty
<100$. A gross estimate of the photon generation rate behind the shock suggests that it is below that required to reach the black body limit, but may be sufficient to enhance the photon-to-baryon ratio and reduce the peak photon energies to the observed values. Further consequences for the properties of the prompt emission are discussed at the end.

The primary dissipation mechanism in jets of gamma-ray bursts (GRBs), and the
high efficiency of the prompt emission are long standing issues. One
possibility is strong collimation of a weakly magnetized relativistic jet by
the surrounding medium, which can considerably enhance the efficiency of the
photospheric emission. We derive a simple analytic criterion for the radiative
efficiency of a collimated jet showing that it depends most strongly on the
baryon loading. We confirm this analytic result by 3D numerical simulations,
and further find that mixing of jet and cocoon material at the collimation
throat leads to a substantial stratification of the outflow as well as sporadic
loading, even if the injected jet is uniform and continuous. One consequence of
this mixing is a strong angular dependence of the radiative efficiency. Another
is large differences in the Lorentz factor of different fluid elements that
lead to formation of internal shocks. Our analysis indicates that in both long
and short GRBs a prominent photospheric component cannot be avoided when
observed within an angle of a few degrees to the axis, unless the asymptotic
Lorentz factor is limited by baryon loading at the jet base to $Gamma_infty
<100$. A gross estimate of the photon generation rate behind the shock suggests
that it is below that required to reach the black body limit, but may be
sufficient to enhance the photon-to-baryon ratio and reduce the peak photon
energies to the observed values. Further consequences for the properties of the
prompt emission are discussed at the end.

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