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To be published in:
Science (Embargoed for Release, 25 MARCH 1999)

Decay of the GRB 990123 Optical Afterglow:
Implications for the Fireball Model.

Alberto J. Castro-Tirado1,2, María Rosa Zapatero-Osorio3, Nicola Caon3, Luz Marina Cairós3,
Jens Hjorth4, Holger Pedersen4, Michael I. Andersen5, Javier Gorosabel1, Corrado Bartolini6,
Adriano Guarnieri6, Adalberto Piccioni6, Filippo Frontera7,8, Nicola Masetti8, Eliana Palazzi8,
Elena Pian8, Jochen Greiner9, Rene Hudec10, Ram Sagar11, A. K. Pandey11, V. Mohan11,
R. K. S. Yadav11, Nilakshi11, Gunnlaugur Björnsson12, Páll Jakobsson12, Ingunn Burud13,
Frédéric Courbin13, Gaetano Valentini14, Anna Piersimoni14, Jesús Aceituno15,
Luz María Montoya15, Santos Pedraz15, Roland Gredel15, Charles F. Claver16, Travis A. Rector16,
James E. Rhoads16, Fabian Walter17, Jürgen Ott18, Hans Hippelein19, Victor Sánchez-Béjar3,
Carlos Gutiérrez3, Alejandro Oscoz3, Jin Zhu20, Jiansheng Chen20, Haotong Zhang20, Jianyan Wei20,
Aiying Zhou20, Sergei Guziy21, Aleksei Shlyapnikov22, John Heise23, Enrico Costa24,
Marco Feroci24 and Luigi Piro24

1 Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, Madrid, Spain
2 Instituto de Astrofísica de Andalucía, IAA-CSIC, Granada, Spain
3 Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
4 Astronomical Observatory, University of Copenhagen, Copenhagen, Denmark
5 Nordic Optical Telescope, NOT, La Palma, Tenerife, Spain
6 Dipartimento di Astronomia, Università di Bologna, Bologna, Italy
7 Dipartimento di Fisica, Università di Ferrara, Ferrara, Italy
8 Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Bologna, Italy
9 Astrophysikalisches Institut, Potsdam, Germany
10 Astronomical Institute, CZ-251 65 Ondrejov, Czech Republic
11 U. P. State Observatory, Manora Park, Naini Tal, 263 129 India
12 Science Institute, University of Iceland, Reykjavik, Iceland
13 Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
14 Osservatorio Astronomico "V. Cerulli", Teramo, Italy
15 Calar Alto Observatory, Almería, Spain
16 Kitt Peak National Observatory, 950 N. Cherry Avenue, P.O. Box 26732, USA
17 Astronomical Observatory, University of Bonn, Germany
18 Radioastronomisches Institut, Auf dem Hügel 71, 53121 Bonn, Germany
19 Max-Plank Institut für Astronomie, Heidelberg, Germany
20 Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China. Beijing Astrophysics Center, Beijing 100871, China
21 Nikolaev University Observatory, Nikolskaya, 24, 327030 Nikolaev, Ukraine
22 Nikolaev Space Recearch Group, Nikolaev Astronomical Observatory, 327030 Nikolaev, Ukraine
23 Space Reseach Organization Netherlands, Utrecht, The Netherlands
24 Istituto di Astrofisica Spaziale, CNR, Frascati


     Multiwavelength, photometric measurements appear as a useful tool, for the understanding of cosmic -ray bursts. Proper modelling would give insight into the expanding fireball, and possibly also to the underlying energy source. Our broad-band (ultraviolet to near-infrared) observations of the exceptionally intense -ray burst, GRB 990123, started ~ 8.5 hours after the event and have continued until 18 Feb 1999. When combined with data from elsewhere, in particular from the Robotic Telescope and Transient Source Experiment (ROTSE) and the Hubble Space Telescope (HST), evidence emerges for a smoothly declining light curve, suggesting some color dependence, that could be related to a cooling break passing the UV-optical band at about 1 d after the high energy event. We also interpret a steepening decline rate seen after 1.5-2 d as evidence for a collimated jet pointing towards the observer.

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