TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2144 SUBJECT: GRB030329, UBVRI polarimetry, R-polarization DATE: 03/04/17 10:47:04 GMT FROM: Alexei Pozanenko at IKI, Moscow Y.Efimov, K.Antoniuk, V.Rumyantsev (CrAO) and A.Pozanenko (IKI) report: We have performed linear polarization observations of the optical counterpart to GRB030329 (Peterson and Price, GCN 1985; Torii, GCN 1986) and a reference star (RA=161.153362, Decl=+21.449663, Henden, GCN 2082) with 125 cm reflector (AZT-11) equipped with double beams UBVRI polarimeter. The observations were done on March 29 between 0.76689 and 0.9230 UT (JD2452728.2669 - 2452728.4230) at moderate sky condition and sometimes variable transparency. Also the reference star appears to be an eclipsing binary (J. Fitzgerald & J. Orosz, GCN 2056, 2070). However it can be considered as a reference star with a sufficient accuracy because of both the simultaneous observation of the star by V.Rumyantsev et al. (GCN 2005) with accuracy better than 0.02mag @ 180 s and orbital period estimation (GCN 2070) show that the star was far from eclipsing phase. The preliminary analysis of UBVRI photometric data shows no significant color variability of the OT, which is in agreement with the prediction of color behavior for "standard" afterglow models. More detailed analysis is in progress. Polarization measurements were corrected for instrumental effects. The weighted mean of reported polarization degree and position angles are derived from the normalized Stokes parameters. The data obtained for OT and reference star are presented below. Indicated in the table are: fractions of Julian Day JD-2452827, filter, polarization parameters (degree P and position angle PA), their mean errors ME, and the duration DT of measurements in minutes. We found that the polarization in R filter may be considered as marginally detected. The interstellar polarization in the direction of GRB 030329 is less then 0.2% according to interstellar polarization data at high galactic latitude reported by Berdyugin et al. (2001,A&A, 372, 276). The polarization of the reference star combined from measurements in two nights (29 and 30 March, 2003) significantly differs from that found for OT of GRB030329. Mid time, (GRB 30329) Date: March, 29, 2003 JD-2452729 FILTER P (%) ME PA ME DT 0.3261 R 0.90 0.39 109 12 86 (Reference star), Combined data March, 29-30, 2003 FILTER P ME PA ME DT R 0.33 0.17 174.2 13.7 Detailed polarization analysis in all UBVRI filters is in progress.