Detection of the optical afterglow of GRB 000630: Implications for dark bursts ^*

¹ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
² European Southern Observatory, Karl-Schwarsschild-Straße 2, 85748 Garching, Germany
³ Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁴ Nordic Optical Telescope, Apartado Postal 474, 38700 Santa Cruz de La Palma, Spain
⁵ Instituto de Astrofísica de Andalucía, CSIC, Apartado Postal 3004, 18080 Granada, Spain
⁶ Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
⁷ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Universities Space Research Association, U. S. Naval Observatory, Flagstaff Station, PO Box 1149, Flagstaff, AZ 86002-1149, USA
⁹ Telescopio Nazionale Galileo, Apartado Postal 565, 38700 Santa Cruz de La Palma, Spain
¹⁰ Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy
¹¹ Department of Physics, University of Tromsø, Tromsø, Norway
¹² NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹³ Universitá di Ferrara, Dipartimento di Fisica, Via Paradiso 12, 44100 Ferrara, Italy
¹⁴ Applied Physics Laboratory, Johns Hopkins University, Laurel MD 20723, USA
¹⁵ University of California, Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450, USA
¹⁶ Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia
¹⁷ Dept. of Physics, Catholic University of America, Washington, DC 20064, USA

Corresponding author: J. U. Fynbo, jfynbo@eso.org

Received: 11 August 2000
Accepted: 12 January 2001

Abstract

We present the discovery of the optical transient of the long-duration gamma-ray burst GRB 000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was . The transient displayed a power-law decline characterized by a decay slope of . A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the transient. The closest detected galaxy is a galaxy 2.0 arcsec north of the transient. The magnitudes of the optical afterglows of GRB 980329, GRB 980613 and GRB 000630 were all less than 24 hours from the burst epoch. We discuss the implications of this for our understanding of GRBs without detected optical transients. We conclude that i) based on the gamma-ray properties of the current sample we cannot conclude that GRBs with no detected OTs belong to another class of GRBs than GRBs with detected OTs and ii) the majority (75% ) of GRBs for which searches for optical afterglow have been unsuccessful are consistent with no detection if they were similar to bursts like GRB 000630 at optical wavelengths.