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Science. 1999 Mar 26;283(5410):2073-5.

Polarimetric constraints on the optical afterglow emission from GRB 990123

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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.


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.

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