Friday, June 21, 2013

1306.4822 (Hirotaka Ito et al.)

Photospheric emission from stratified jets    [PDF]

Hirotaka Ito, Shigehiro Nagataki, Masaomi Ono, Shiu-Hang Lee, Jirong Mao, Shoichi Yamada, Asaf Pe'er, Akira Mizuta, Seiji Harikae
We explore photospheric emissions from stratified two-component jets, wherein a highly relativistic spine outflow is surrounded by a wider and less relativistic sheath outflow. The propagation of thermal photons which are injected in regions of high optical depth is solved until they escape at the photosphere. Due to the presence of shear in velocity (Lorentz factor) at the boundary of the spine and sheath region, fraction of the injected photons are accelerated via a Fermi-like acceleration mechanism such that a high energy power-law tail is formed in the resultant spectrum. We show, in particular, that if a velocity shear with a considerable variance in the bulk Lorentz factor is present, the high energy part of observed Gamma-ray Bursts (GRBs) photon spectrum can be explained by this photon acceleration mechanism. We also show that the accelerated photons may also account for the origin of the extra hard power-law component above the bump of the thermal-like peak seen in some peculiar burst (e.g., GRB 090510, 090902B, 090926A). It is demonstrated that time-integrated spectra can also reproduce the low energy spectrum of GRBs consistently due to a multi-temperature effect when time evolution of the outflow is considered. Finally, we show that the empirical Ep-Lp relation can be explained by the difference in the outflow properties of individual sources.
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