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Issue A&A
Volume 410, Number 3, November II 2003
Page(s) 823 - 831
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20031321



A&A 410, 823-831 (2003)
DOI: 10.1051/0004-6361:20031321

Intrinsic and dust-induced polarization in gamma-ray burst afterglows: The case of GRB 021004

D. Lazzati1, S. Covino2, S. di Serego Alighieri3, G. Ghisellini2, J. Vernet3, E. Le Floc'h4, D. Fugazza5, S. Di Tomaso5, D. Malesani6, N. Masetti7, E. Pian8, E. Oliva5 and L. Stella9

1  Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
2  INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
3  INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
4  Service d'Astrophysique, C.E. Saclay, 91191 Gif-sur-Yvette Cedex, France
5  INAF - Telescopio Nazionale Galileo, Roque de los Muchachos, PO Box 5653, 38700 Santa Cruz de la Palma, Spain
6  International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
7  Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, via Gobetti 101, 40129 Bologna, Italy
8  INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
9  INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy

(Received 11 June 2003 / Accepted 19 August 2003)

Abstract
Polarization measurements for the optical counterpart to GRB 021004 are presented and discussed. Our observations were performed with the TNG and the VLT-UT3 (Melipal) during the first and fourth night after the gamma-ray burst discovery. We find robust evidence for temporal evolution of the polarization, which is therefore, at least partially, intrinsic to the optical transient. We do not find convincing evidence of wavelength dependence for the intrinsic polarization of the transient, in agreement with current polarization models for optical afterglows. We discuss the role of dust, both in our galaxy and in the host, in modifying the transmitted polarization vector, showing how a sizable fraction of the observed polarized flux is due to Galactic selective extinction, while it is not possible to single out any clear contribution from dust in the host galaxy. We discuss how our data compare to those obtained by different groups showing that a two-component model is required to describe the complete dataset. This is not surprising given the complex lightcurve of GRB 021004.


Key words: gamma rays: bursts -- polarization -- ISM: dust, extinction -- radiation mechanisms: non-thermal

Offprint request: D. Lazzati, lazzati@ast.cam.ac.uk

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© ESO 2003


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