Jens Hjorth, ^1* Gunnlaugur Björnsson, ² Michael I. Andersen, ³ Nicola Caon, ⁴ Luz Marina Cairós, ⁴ Alberto J. Castro-Tirado, ⁵⁶ María Rosa Zapatero Osorio, ⁴ Holger Pedersen, ¹ Enrico Costa ⁷

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.

¹ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark.
² Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland.
³ Nordic Optical Telescope, Ap. 474 St. Cruz de La Palma, E-38700 Canarias, Spain.
⁴ Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain.
⁵ Laboratorio de Astrofísica Espacial y Física Fundamental, Instituto Nacional de Técnica Aeroespacial, P.O. Box 50727, E-28080 Madrid, Spain.
⁶ Instituto de Astrofísica de Andalucía, Consejo Superior de Investigaciones Científicas, P.O. Box 03004, E-18080 Granada, Spain.
⁷ Istituto di Astrofisica Spaziale-C.N.R., Via Fosso del Cavaliere, I-00133 Rome, Italy.
^*   To whom correspondence should be addressed. E-mail: jens{at}astro.ku.dk