{
  "bibcode": "2002GCN..1498....1C",
  "body": "S. Covino, D. Malesani, G. Ghisellini, P. Saracco, G. Tagliaferri, F.\nZerbi (Observ. of Brera, Milan, Italy); S. Di Serego, A. Cimatti, M.\nDella Valle (Observ. of Arcetri, Florence, Italy); F. Fiore, G.L.\nIsrael, L. Stella (Observ. of Monte Porzio, Rome, Italy); M. Vietri\n(Univ. Rome 3, Italy); N.  Kawai (Tokyo Tech, Japan); D. Lazzati (IoA,\nCambridge, UK); S. Ortolani (Univ.  of Padua, Italy); L. Pasquini (ESO,\nGermany); G. Ricker (MIT, USA); E. Le Floch, P. Goldoni, F. Mirabel\n(CEA, France) report:\n\n\nWe observed the optical counterpart to GRB020813 (Villasenor et al., GCN\n1471; Fox, Blake & Drake, GCN 1470) between August 13.994 and August\n14.148 (22.9 to 24.8 hours after the GRB trigger). The observations were\nperformed with the ESO VLT-3 (Melipal) telescope equipped with FORS1\nwith a Bessel V band filter in the imaging polarimetry mode. The\ntransient source is clearly detected in the acquisition image.\n\nWe found linear polarization at the level of P = (1.17 +/- 0.16)% with\nposition angle (158 +/- 4) degrees (uncertanties are 1-sigma errors).\n\nOnce we also remove the effect of ISM in our Galaxy measuring the\npolarization of field stars, the intrinsic polarization of the optical\ntransient turns out to be P = (0.80 +/- 0.16)% and P.A.= (144 +/- 6)\ndegrees.\n\nThe detected polarization level is significantly lower (~6 sigma) than\nthe previous one measured by Barth et al. (GCN 1477) at UT 13.31 while\nthe position angle remains constant to within the errors.\n\nThis is therefore the most significant and larger degree of variability\never detected in the polarization level of a GRB optical afterglow and\nconfirms that the detected polarization is related to the afterglow\nemission and not induced by intervening matter.\n\nSuch a degree of variability is predicted in current models for beamed\nafterglows (Ghisellini & Lazzati, 1999, MNRAS, 309, L7) and allows us to\npredict that the jet break occurred ~one day after the GRB event.\nAccording to these models, the polarization angle should then rotate by\n90 degrees and reach a peak up to ~10% around Aug 17.0 UT. Further\npolarization measurements are strongly urged to test such models.\n\nThis message is citeable.",
  "circularId": 1498,
  "createdOn": 1029512592000,
  "email": "malesani@merate.mi.astro.it",
  "subject": "Polarimetry of GRB020813: evidence for variability",
  "submitter": "Daniele Malesani at Obs.Astro. di Brera  <malesani@merate.mi.astro.it>",
  "eventId": "GRB 020813"
}