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TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2167 SUBJECT: GRB 030329: optical spectropolarimetry at VLT DATE: 03/04/24 18:55:20 GMT FROM: Daniele Malesani at SISSA-ISAS,Trieste,Italy <malesani@sissa.it> S. Covino, G. Ghisellini (INAF, Brera, I); D. Malesani (SISSA, Ts, I); P.A. Price (RSAA, ANU); D. Lazzati, E.M. Rossi (IoA, Cambridge, UK); G. Chincarini, G. Tagliaferri, F.M. Zerbi (INAF, Brera, I); A. Cimatti, S. Di Serego, M. Della Valle (INAF, Arcetri, I); F. Fiore, G.L. Israel, L. Stella (INAF, Roma, I); M. Vietri (Scuola Norm. Sup., Pisa, I); N. Kawai (Tokyo Tech, Japan); G.R. Ricker (MIT, USA); P. Goldoni, E. Le Floc'h, I.F. Mirabel (CEA, Sacley, F); S. Mereghetti (IASF, Mi, I); E. Costa, P. Soffitta (IASF, Roma, I); S. Ortolani (Padova Univ., I); A.O. Jaunsen, A. Kaufer, A. Lopez, P. Vaisanen, P.M. Vreeswijk (ESO); report: Starting on 2003 April 2.1 (3.6 days after the GRB), we observed the optical counterpart (Peterson & Price, GCN 1985; Torii, GCN 1986) of GRB 030329 (Vanderspek et al., GCN 1997). The observations were performed with the ESO VLT-UT1 (Antu) telescope equipped with FORS1, in spectropolarimetric mode. At that time, the afterglow had a magnitude V ~ 17.7 (Covino et al., GCN 2122). Our spectrum spans the wavelenght range 350-850 nm. Several narrow emission lines are clearly detected (e.g. Greiner et al., GCN 2020). The polarization level is found very small, yet significantly nonzero, decreasing from (0.9+/-0.1)% in the blue to (0.5+/-0.1)% in the red part of the spectrum (1-sigma errors); the polarization angle also shows a mildly significant variation from (83+/-4) deg in the blue to (73+/-5) deg in the red. These trends are consistent with the effect of a mildly polarizing Galactic/host ISM on a intrinsically polarized afterglow. In particular, the values we obtain in correspondence of the R-band (lambda ~ 650 nm) are P = (0.5+/-0.1)% and theta = (73+/-5) deg. Comparison of our result with previous measurements (Efimov et al., GCN 2144; Magalhães et al., GCN 2163) might indicate some time variability on a few-days timescale. We thank the ESO staff at Paranal for their kind and reliable assistance. This message may be cited.
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