TITLE:   GCN CIRCULAR
NUMBER:  2144
SUBJECT: GRB030329, UBVRI polarimetry, R-polarization
DATE:    03/04/17 10:47:04 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

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.