Physics of the GRB 030328 afterglow and its environment

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: maiorano@iasfbo.inaf.it
² INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
³ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40126 Bologna, Italy
⁴ The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁵ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
⁶ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁷ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁸ Research School of Astronomy and Astrophysics, Australian National University, via Cotter Road, Weston, ACT 2611, Australia
⁹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
¹⁰ Astrophysikalisches Institut, 14482 Potsdam, Germany
¹¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹² Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Universities Space Research Association / US Naval Observatory, PO Box 1149, Flagstaff, AZ 86002, USA
¹⁵ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
¹⁶ NASA MSFC, SD-50, Huntsville, AL 35812, USA
¹⁷ Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Rome, Italy
¹⁸ European Southern Observatory, Karl Schwarzschild-Strasse 2, 85748 Garching, Germany
¹⁹ School of Physics and Astronomy and the Wise Observatory, University of Tel-Aviv, Tel-Aviv 69978, Israel
²⁰ ARIES Observatory, Manora Peak, Naini Tal, 263129 Uttaranchal, India
²¹ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
²² Institute of Astronomy “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 20 December 2005
Accepted: 19 March 2006

Abstract

Aims.To investigate the physical nature of the afterglow emission. We report on the photometric, spectroscopic and polarimetric observations of the optical afterglow of Gamma–Ray Burst (GRB) 030328 detected by HETE-2.

Methods.Photometric, spectroscopic and polarimetric monitoring of the optical afterglow.

Results.Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices  ± 0.03,  ± 0.07 and a break at t_b = 0.48 ± 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle 32. An achromatic bump starting around ~0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 ± 0.0006 and at z = 1.295 ± 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-α Absorber. The optical V-band afterglow appears polarized, with P = (2.4 ± 0.6)% and θ =  ± , suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope  ± 0.15, and an X-ray slope  ± 0.2.

Conclusions.The discussion of the results in the context of the “fireball model” shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.