The bright optical afterglow of the long GRB 001007 ^*

¹ Real Instituto y Observatorio de la Armada, Sección de Astronomía, 11.110 San Fernando-Naval (Cádiz), Spain e-mail: josemari@alumni.nd.edu
² Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos, 3.004, 18.080 Granada, Spain, e-mail: ajct@iaa.es, jgu@iaa.es
³ Danish Space Research Institute, Juliane Maries Vej 30, 2 100 Copenhagen Ø, Denmark, e-mail: jgu@dsri.dk
⁴ Laboratorio de Astrofísica Espacial y Física Fundamental (INTA), Apartado de Correos, 50.727, 28.080 Madrid, Spain e-mail: jgu@laeff.esa.es
⁵ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2 100 Copenhagen Ø, Denmark e-mail: jens@astro.ku.dk, brian_j@astro.ku.dk, holger@astro.ku.dk
⁶ European Southern Observatory, Karl Schwarzschild Straße 2, 85 748 Garching, Germany, e-mail: jfynbo@eso.org
⁷ Division of Astronomy, PO Box 3 000, 90 014 University of Oulu, Finland, e-mail: michael.andersen@oulu.fi
⁸ Instituto de Astrofísica de Canarias, 38.200 La Laguna (Tenerife), Spain e-mail: martinlc@ll.iac.es, jcv@ll.iac.es, yves@ll.iac.es
⁹ Departamento de Ciencias de la Navegación y de la Tierra, Universidad de la Coruña, 15.011 La Coruña, Spain e-mail: olga@mail2.udc.es
¹⁰ Department of Physics, Michigan Technological University, 1 400 Townsend Drive, Houghton MI 49 931-1 295, USA e-mail: dperez@mtu.edu
¹¹ School of Physics and Astronomy, University of Nottingham, University Park NG7 2RD Nottingham, UK, e-mail: milvang@astro.ku.dk
¹² Department of Astronomy, University of Toronto, 60 St. George Street, Toronto ON, M5S 3H8 Canada e-mail: mallen@astro.utoronto.ca
¹³ Space Telescope Science Institute, 3 700 San Martín Drive, Baltimore MD 21 218, USA, e-mail: fruchter@stsci.edu
¹⁴ Astrophysikalisches Institut, An der Sternwarte 16, 14 482 Potsdam, Germany, e-mail: jgreiner@aip.de
¹⁵ Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34 131 Trieste, Italy, e-mail: pian@tesre.bo.cnr.it
¹⁶ University of Amsterdam, Kruislaan 403, 1 098 SJ Amsterdam, The Netherlands e-mail: pmv@astro.uva.nl, lexk@astro.uva.nl, evert@astro.uva.nl, isabel@astro.uva.nl, edvdh@astro.uva.nl
¹⁷ Goddard Space Flight Centre (NASA), Greenbelt MD 20 771 USA e-mail: scott@milkyway.gsfc.nasa.gov, thomas.cline@gsfc.nasa.gov, jack.trombka@gsfc.nasa.gov
¹⁸ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna (CNR), Via Gobetti 101, 40 129 Bologna, Italy e-mail: filippo@tesre.bo.cnr.it, masetti@tesre.bo.cnr.it, eliana@tesre.bo.cnr.it
¹⁹ Thüringer Landessternwarte Tautenburg, 07 778 Tautenburg, Germany, e-mail: klose@tls-tautenburg.de
²⁰ Marshall Space Flight Centre (NASA), SD-50, Huntsville AL 35 812, USA, e-mail: kouveliotou@eagles.msfc.nasa.gov
²¹ Clemson University, Clemson SC 29 634, USA, e-mail: hartmann@grb.phys.clemson.edu
²² University of California, Berkeley, Space Sciences Laboratory, Berkeley CA 94 720-7 450, USA, e-mail: khurley@ssl.berkeley.edu
²³ Ioffe Physico-Technical Institute, 26 Polytekhnicheskaya, St. Petersburg 194 021, Russia, e-mail: mazets@astro.ioffe.rssi.ru
²⁴ Lawrence Livermore National Laboratory, Livermore CA 94 550, USA, e-mail: park@ursa.llnl.gov
²⁵ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK e-mail: nrt@star.herts.ac.uk
²⁶ Department of Physics and Astronomy, State University of New York, Stony Brook NY 11 794-3800, USA e-mail: rwijers@astro.sunysb.edu
²⁷ Steward Observatory, Tucson AZ 85 721, USA, e-mail: gwilliams@as.arizona.edu

Corresponding author: J. M. Castro Cerón, josemari@alumni.nd.edu

Received: 25 March 2002
Accepted: 8 July 2002

Abstract

We present optical follow up observations of the long GRB 001007 between 6.14 hours and ~468 days after the event. An unusually bright optical afterglow (OA) was seen to decline following a steep power law decay with index α = -2.03 ± 0.11, possibly indicating a break in the light curve at 3.5 days, as found in other bursts. Upper limits imposed by the LOTIS alerting system 6.14 hours after the gamma ray event provide tentative (1.2σ) evidence for a break in the optical light curve. The spectral index β of the OA yields -1.24 ± 0.57. These values may be explained both by several fireball jet models and by the cannonball model. Fireball spherical expansion models are not favoured. Late epoch deep imaging revealed the presence of a complex host galaxy system, composed of at least two objects located 1.2´´ (1.7σ) and 1.9´´ (2.7σ) from the afterglow position.