A theory of photospheric emission from relativistic, collimated outflows

By C. Lundman, A. Pe’er, F. Ryde. 2012

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Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle-dependent velocity profiles, Γ=Γ(r, Θ, φ), where Γ is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Γ ≈ Γ0/[(Θ/Θj)p + 1], where Θj is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, Θv. We show that for narrow jets (ΘjΓ0 l few), the obtained low-energy photon index is α ≈-1 (dN/dE / Eα), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at Θv ≈ Θj. This result is surprisingly similar to the average low-energy photon index seen in gamma-ray bursts. For wide jets (ΘjΓ0 g few) observed at Θv g Θj, a multicolour blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/e4d9b7655f1cf95fd1f7e79156bb02dcb97ee69d

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In Astronomy and Astrophysics

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In Nuclear and High Energy Physics

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