GCN Circular 1595

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),  M. Dahlem, P. Vaisanen, C. 
Melo, C.  Lidman (ESO Paranal) report:


We observed the optical counterpart to GRB 021004 (Shirasaki et al. GCN 1565
Not found
 
 
; 
Fox, GCN 1564
Not found
 
 
) on October 5.077 (13.75 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 in the acquisition image with a 
magnitude V = 19.34 +/- 0.02 with respect to the USNO star at 00:26:58.713 
+18:56:56.61(Fox, GCN 1564
Not found
 
 
) as calibrated by Henden (GCN 1583
Not found
 
 
).

The polarimetric observations were obtained from October 5.080 to 5.149 UT 
(from 13.82 to 15.48 hours after the burst). We found linear polarization at 
the level of P = (1.26 +/- 0.10)% with position angle (114.2 +/- 2.2) degrees 
(uncertanties are 1-sigma errors).

The polarization induced by the interstellar matter in our Galaxy was 
estimated by field stars and accounts to P = (0.61 +/- 0.04)% with position 
angle (114.4 +/- 1.7) degrees. Once we remove the effect of ISM the intrinsic 
polarization of the optical transient turns out to be P = (0.65 +/- 0.11)% 
and position angle (114.4 +/-  4.6) degrees. Given that the field stars show 
a rather wide distribution in polarization degree and that the position angle 
for the stars and the optical transient are essentially the same, we can not 
rule out the possibility that the measured polarization is entirely due to 
the ISM in the Galaxy. This, in addition, leaves little room for host galaxy 
ISM induced polarization, that could account for at most a small fraction of 
the observed polarization. 


At the web page http://www.merate.mi.astro.it/~malesani/GRB021004 we report 
the QU Stokes' parameter plane.



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