{
  "bibcode": "2003GCN..2163....1M",
  "body": "A.M. Magalh�es, A. Pereyra, T. Dominici and Z. Abraham (IAG, U.\nof S�o Paulo) report:\n\nWe have obtained R band imaging polarimetry of GRB030329. The\nobservation was performed with the IAGPOL imaging polarimeter and the\nIAG-USP 60cm telescope, at the Laborat�rio Nacional de Astrof�sica site.\nThe observation spanned the period UT 01:06 to 01:51 on 31 March,\n1.519 - 1.592 days after the event, and was made through less than ideal\nsky conditions. The double-beam technique provided by a calcite plate\nanalyser ensured however that useful polarimetric data could still be\nobtained.\n\nWe have measured the following linear polarization for GRB030329:\n\nP = [1.974 +- 0.477] %, position angle = 83.2 degrees.\n\n(The position angle is already in the equatorial system, increasing from\nNorth to East.)\n\nAs a comparison, we have obtained much smaller polarization for field\nobjects. In particular, for USNO A-2.0 1050-6351075 (object 'A' in\nRumyantsev et al, GCN 2005) and 1050-6349885, we have obtained the\nfollowing linear polarization values: [0.58 +- 0.11] at 66deg and [0.06\n+- 0.12] at 21deg, respectively. This low foreground polarization is\nconsistent with the very low foreground reddening towards the field\n(E(B-V)=0.025, Schlegel et al. 1998).\n\nDilution of the GRB intrinsic polarization by the much fainter (Blake &\nBloom GCN 2011) host galaxy should be negligible. Polarization within\nthe host galaxy interstellar medium seems unlikely due to the small\ninternal extincion (Caldwell et al. GCN 2053); this should be settled\nby variability studies and spectropolarimetry.\n\nWe conclude that a substantial fraction of the GRB030329 polarization\nis intrisic in nature. This strongly suggests that GRB030329 is\nnon-spherically symmetric, lending further credence to the asymmetric\nfireball scenario (in combination with a physical process that produces\npolarization, such as synchtron radiation). The position angle above\nshould be correlated with the object's beam or jet orientation on the\nsky. It would be interesting to see whether a similar position angle\nis present in forthcoming resolved radio observations of the source.\n\nThis message may be cited.\n\n[GCN OPS NOTE (22Apr03):  The circular was originally submitted\non 14 Apr 03, but was delayed due to email address mismatch.]",
  "circularId": 2163,
  "createdOn": 1051044648000,
  "email": "antonio@astro.iag.usp.br",
  "subject": "GRB030329: optical polarization",
  "submitter": "Antonio Pereyra at IAG-U.Sao Paulo  <antonio@astro.iag.usp.br>",
  "eventId": "GRB 030329"
}