GCN Circular 1498

S. Covino, D. Malesani, G. Ghisellini, P. Saracco, G. Tagliaferri, F.
Zerbi (Observ. of Brera, Milan, Italy); S. Di Serego, A. Cimatti, M.
Della Valle (Observ. of Arcetri, Florence, Italy); F. Fiore, G.L.
Israel, L. Stella (Observ. of Monte Porzio, Rome, Italy); M. Vietri
(Univ. Rome 3, Italy); N.  Kawai (Tokyo Tech, Japan); D. Lazzati (IoA,
Cambridge, UK); S. Ortolani (Univ.  of Padua, Italy); L. Pasquini (ESO,
Germany); G. Ricker (MIT, USA); E. Le Floch, P. Goldoni, F. Mirabel
(CEA, France) report:


We observed the optical counterpart to GRB020813 (Villasenor et al., GCN
1471
Not found
 
 
; Fox, Blake & Drake, GCN 1470
Not found
 
 
) between August 13.994 and August
14.148 (22.9 to 24.8 hours after the GRB trigger). The observations were
performed with the ESO VLT-3 (Melipal) telescope equipped with FORS1
with a Bessel V band filter in the imaging polarimetry mode. The
transient source is clearly detected in the acquisition image.

We found linear polarization at the level of P = (1.17 +/- 0.16)% with
position angle (158 +/- 4) degrees (uncertanties are 1-sigma errors).

Once we also remove the effect of ISM in our Galaxy measuring the
polarization of field stars, the intrinsic polarization of the optical
transient turns out to be P = (0.80 +/- 0.16)% and P.A.= (144 +/- 6)
degrees.

The detected polarization level is significantly lower (~6 sigma) than
the previous one measured by Barth et al. (GCN 1477
Not found
 
 
) at UT 13.31 while
the position angle remains constant to within the errors.

This is therefore the most significant and larger degree of variability
ever detected in the polarization level of a GRB optical afterglow and
confirms that the detected polarization is related to the afterglow
emission and not induced by intervening matter.

Such a degree of variability is predicted in current models for beamed
afterglows (Ghisellini & Lazzati, 1999, MNRAS, 309, L7) and allows us to
predict that the jet break occurred ~one day after the GRB event.
According to these models, the polarization angle should then rotate by
90 degrees and reach a peak up to ~10% around Aug 17.0 UT. Further
polarization measurements are strongly urged to test such models.

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