2012/08/31 by Christoffer Lundman, Asaf Pe’er, Asaf Pe'er +2 · 4 citations
Physics and Astronomy · #Approx #Astrophysics #Astrophysics and Cosmic Phenomena #Collimated light #Gamma-ray burst #Gamma-ray bursts and supernovae #Jet (fluid) #Lorentz factor #Lorentz transformation #Photon #Physics #Pulsars and Gravitational Waves Research #Quantum mechanics #Spectral index #Spectral line #astro-ph.HE
paper · pdf · doi:10.1093/mnras/sts219
15 pages, 11 figures; Minor changes in latest version, accepted for publication in MNRAS
openalex publication_date 2012/11/08 · arxiv created 2012/11/29 · arxiv updated 2015/06/11 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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 ≲ 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 ≳ few) observed at θv ≪ 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.