2007/09/30 by Kenji Toma, Kunihito Ioka, Takashi Nakamura · 4 citations
Physics and Astronomy · #Afterglow #Astrophysics and Cosmic Phenomena #Coupling (piping) #Electron #Faraday effect #Gamma-ray burst #Gamma-ray bursts and supernovae #Linear polarization #Magnetic field #Polarization (electrochemistry) #Pulsars and Gravitational Waves Research #Thermal #astro-ph
paper · pdf · doi:10.1086/528740
published as Astrophys.J. 673 (2008) L123-L126 · 4 pages, 2 figures, title revised, discussions of section 3 and 5 expanded. Accepted for publication in ApJL. Replaced with the accepted version
arxiv created 2007/12/18 · openalex publication_date 2007/12/31 · arxiv updated 2015/04/24 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
The late-time optical/radio afterglows of γ-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources. However, the fraction f of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind. If f < 1, the true explosion energies of GRBs are f −1 times larger than those commonly estimated with f = 1. Thus the value of f gives an important constraint on the nature of the central engine of GRBs and the physics of collisionless shocks. Although early-time radio observations can probe the thermal electrons, they are difficult at present. We show that the Faraday rotation effects of the thermal electrons may suppress the linear polarization of the afterglow at frequencies higher than the absorption frequency in the late time, if the magnetic field is ordered at least in parts, and that f can be constrained through observation of the effects. We find that these effects may be detected with late-time, ≥1 day, polarimetry with ALMA for a burst occurring within 1 Gpc (i.e., z ≃ 0.2), if f ∼ 10 −1 .