S. Covino, G. Ghisellini, D. Malesani, G. Tagliaferri, F. Zerbi (INAF Brera,
Italy); A. Cimatti, M. Della Valle, S. Di Serego (INAF Arcetri, Italy); F.
Fiore, G.L. Israel, L. Stella (INAF Roma, Italy); E. Costa, P. Soffitta (IASF
Roma, Italy), S. Mereghetti (IASF Milano, Italy), N. Kawai (Tokyo Tech,
Japan); D. Lazzati, E.M. Rossi (IoA, Cambridge, UK); S. Ortolani (Univ. of
Padova, Italy); L. Pasquini (ESO, Germany); G. Ricker (MIT, USA); E. Le
Floch, P. Goldoni, F. Mirabel (CEA, France), C. Lidman, G. Marconi, C. Melo,
P. Vaisanen (ESO Paranal), B. Aracil (IAP, France) report:
We observed the optical counterpart to GRB 021004 (Shirasaki et al. GCN 1565;
Fox, GCN 1564) on October 8.225 (89.3 hours after the GRB trigger). The
observations were performed with the ESO VLT-UT3 (Melipal) telescope equipped
with FORS1 and a Bessel V filter in the imaging polarimetry mode. The
transient source is clearly detected with a magnitude V = 20.89 +/- 0.03 with
respect to the USNO star at 00:26:58.713 +18:56:56.61 (Fox, GCN 1564) as
calibrated by Henden (GCN 1583).
The polarimetric observations allowed us to single out linear polarization at
the level of P = (0.67 +/- 0.23)% with position angle (89.0 +/- 10.0) degrees
(uncertainties are 1-sigma errors) for the optical transient. The field star
polarization was of course constant within the errors between our two
polarimetric observations (Covino et al. GCN 1595).
Once we remove the estimated polarization induced by the interstellar matter
(ISM) in our Galaxy, as discussed in our GCN 1595, the intrinsic polarization
of the optical transient turns out to be consistent with zero.
Independently of any ISM induced polarization, the present result compared to
the polarimetric observations performed on October 5.115 [Covino et al. GCN
1595, P = (1.26 +/- 0.1)%, angle = (114.2 +/- 2.2) degrees] and 5.175 [Rol et
al. GCN 1596, P = (1.32 +/- 0.28)%, angle = (125 +/- 1) degrees] could show
a marginal hint for a change of the polarization degree and of the position
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