Author: Christoffer Lundman
Abstract: Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle-dependent velocity profiles. We consider here photospheric emission from various jet profiles. In collimated jets, the observed spectrum depends on the viewing angle. We show, both analytically and by simulations, that for a large parameter space region the obtained low energy spectral slope is approximately flat (FE ∝ E0). This result is not sensitive to the viewing angle or angular Lorentz factor gradient. The obtained spectral index is surprisingly similar to the average spectral index seen in gamma-ray bursts. For wide jets (θj >> 1/Γ) viewed head on, a multicolor blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.