Polarimetric constraints on the optical afterglow emission from GRB 990123
Polarization of the optical emission from GRB 990123 was measured on 24.17 January 1999 universal time with the Nordic Optical Telescope. An upper limit of 2.3% on the linear polarization was found. Accurate polarization measurements provide important clues to the blast wave geometry and magnetic field structure of gamma-ray bursts (GRBs). The lack of detectable polarization for GRB 990123 indicates that the optical afterglow was produced by a blast wave of unknown geometry with an insignificant coherent magnetic field or by a beamed outflow at high Lorentz factor seen at a small viewing angle. Such a collimated jet would help solve the problem of energy release in this exceptionally luminous cosmological burst.
Hjorth
J
Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen O, Denmark. Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland. Nordic Optical Telescope, Ap. 474 St. Cruz de La Palma, E-387.
Bjornsson
G
Andersen
MI
Caon
N
Cairos
LM
Castro-Tirado
AJ
Osorio
MR
Pedersen
H
Costa
E