TITLE: GCN CIRCULAR NUMBER: 1622 SUBJECT: GRB 021004: Additional V-band Polarimetric Observations DATE: 02/10/11 08:54:25 GMT FROM: Stefano Covino at Brera Astronomical Observatory 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 angle. This message is citeable.