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;
Fox, GCN 1564) 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) as calibrated by Henden (GCN 1583).
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.
This message is citeable.