Michael I. Andersen, ^1* Alberto J. Castro-Tirado, ²³ Jens Hjorth, ⁴ Palle Møller, ⁵ Holger Pedersen, ⁴ Nicola Caon, ⁶ Luz Marina Cairós, ⁶ Heidi Korhonen, ¹⁷ María Rosa Zapatero Osorio, ⁶ Enrique Pérez, ³ Filippo Frontera ⁸

An optical spectrum of the afterglow from the unusually bright gamma-ray burst GRB 990123 obtained on 24.25 January 1999 universal time showed an absorption system at a redshift of z = 1.600. The absence of a hydrogen Lyman  forest sets an upper limit of z < 2.17, whereas ultraviolet photometry indicates an upper limit of z < 2.05. The probability of intersecting an absorption system as strong as the one observed along a random line of sight out to this z is at most a few percent, implying that GRB 990123 was probably at z = 1.600. Currently favored cosmological parameters imply that an isotropic energy release equivalent to the rest mass of 1.8 neutron stars (4.5 × 10⁵⁴ erg) was emitted in gamma rays. Nonisotropic emission, such as intrinsic beaming, may resolve this energy problem.

¹ Nordic Optical Telescope, Apartado 474 St. Cruz de La Palma, E-38700 Canarias, Spain.
² Laboratorio de Astrofísica Espacial y Física Fundamental, Instituto Nacional de Técnica Aeroespacial, Post Office Box 50727, E-28080 Madrid, Spain.
³ Instituto de Astrofísica de Andalucía, Consejo Superior de Investigaciones Científicas, Post Office Box 03004, E-18080 Granada, Spain.
⁴ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark.
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany.
⁶ Instituto de Astrofísica de Canarias, E-38200, La Laguna, Tenerife, Spain.
⁷ Astronomy Division, University of Oulu, Post Office Box 3000, FIN-90401 Oulu, Finland.
⁸ Dipartimento di Fisica, Universitá di Ferara, Ferara, Italy.
^*   To whom correspondence should be addressed. E-mail: andersen{at}not.iac.es