TITLE:   GCN CIRCULAR
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; Magalh�es 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.

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