TITLE:   GCN GRB OBSERVATION REPORT
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  <covino@merate.mi.astro.it>

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.   



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