GRB 021004: polarimetric observations
E. Rol (University of Amsterdam), J.M. Castro Cer�n (ROA, San Fernando),
J. Gorosabel, A.J. Castro-Tirado (IAA-CSIC, Granada), A. Fruchter (STScI,
Baltimore), J. Greiner (MPE, Garching), J. Hjorth (Univ. of Copenhagen),
E. Pian (OAT-INAF, Trieste) R.A.M. Wijers and E. van den Heuvel (Univ.
of Amsterdam), on behalf of GRACE, report:
"We have performed polarimetric observations of the afterglow of
GRB 021004 (Fox, GCN 1564; Shirasaki et al. GCN 1565) with the FORS1
instrument at the VLT. The observations were done between October 5.15
and October 5.20 UT in the V-band.
Independently of Covino et al. (GCN 1595), we find a polarization value
for the OT of P = (1.32 +/- 0.28) %, with a position angle of theta =
(125 +/- 1) degrees.
Correcting for possible polarization due to the interstellar medium is
rather difficult, since there are few bright stars close to the OT position.
Using 12 bright stars within 4.5 arcmin from this position, we find a mean
ISM polarization of P = (0.7 +/- 0.3) % and theta = (116 +/- 4) degrees.
The error in the degree of polarization is obtained from the spread in the
values found for the field stars.
The resultant degree of polarization for the OT is P = (0.57 +/- 0.40) %.
Since this result could be entirely due to polarization induced by ISM,
we derive a 3-sigma upper limit of P = 1.2 % for the degree of
The QU Stokes' parameter plane can be seen at:
This note can be cited."