Polarimetric Constraints on the Optical Afterglow Emission from GRB 990123
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
