{
  "bibcode": "2002GCN..1595....1C",
  "body": "S. Covino, G. Ghisellini, D. Malesani, G. Tagliaferri, F. Zerbi (INAF Brera, \nItaly); A. Cimatti, M. Della Valle, S. Di Serego (INAF Arcetri, Italy); F. \nFiore, G.L. Israel, L. Stella (INAF Roma, Italy); E. Costa, P. Soffitta (IASF \nRoma, Italy), S. Mereghetti (IASF Milano, Italy), N. Kawai (Tokyo Tech, \nJapan); D. Lazzati, E.M. Rossi (IoA, Cambridge, UK); S. Ortolani (Univ. of \nPadova, Italy); L. Pasquini (ESO, Germany); G. Ricker (MIT, USA); E. Le \nFloch, P. Goldoni, F. Mirabel (CEA, France),  M. Dahlem, P. Vaisanen, C. \nMelo, C.  Lidman (ESO Paranal) report:\n\n\nWe observed the optical counterpart to GRB 021004 (Shirasaki et al. GCN 1565; \nFox, GCN 1564) on October 5.077 (13.75 hours after the GRB trigger). The \nobservations were performed with the ESO VLT-UT3 (Melipal) telescope equipped \nwith FORS1 and a Bessel V filter in the imaging polarimetry mode. The \ntransient source is clearly detected in the acquisition image with a \nmagnitude V = 19.34 +/- 0.02 with respect to the USNO star at 00:26:58.713 \n+18:56:56.61(Fox, GCN 1564) as calibrated by Henden (GCN 1583).\n\nThe polarimetric observations were obtained from October 5.080 to 5.149 UT \n(from 13.82 to 15.48 hours after the burst). We found linear polarization at \nthe level of P = (1.26 +/- 0.10)% with position angle (114.2 +/- 2.2) degrees \n(uncertanties are 1-sigma errors).\n\nThe polarization induced by the interstellar matter in our Galaxy was \nestimated by field stars and accounts to P = (0.61 +/- 0.04)% with position \nangle (114.4 +/- 1.7) degrees. Once we remove the effect of ISM the intrinsic \npolarization of the optical transient turns out to be P = (0.65 +/- 0.11)% \nand position angle (114.4 +/-  4.6) degrees. Given that the field stars show \na rather wide distribution in polarization degree and that the position angle \nfor the stars and the optical transient are essentially the same, we can not \nrule out the possibility that the measured polarization is entirely due to \nthe ISM in the Galaxy. This, in addition, leaves little room for host galaxy \nISM induced polarization, that could account for at most a small fraction of \nthe observed polarization. \n\n\nAt the web page http://www.merate.mi.astro.it/~malesani/GRB021004 we report \nthe QU Stokes' parameter plane.\n\n\n\nThis message is citeable.",
  "circularId": 1595,
  "createdOn": 1033983187000,
  "email": "covino@merate.mi.astro.it",
  "subject": "GRB 021004: V-band Polarimetric Observations",
  "submitter": "Stefano Covino at Brera Astronomical Observatory  <covino@merate.mi.astro.it>",
  "eventId": "GRB 021004"
}