GCN Circular 2163
Subject
GRB030329: optical polarization
Event
Date
2003-04-22T20:50:48Z (23 years ago)
From
Antonio Pereyra at IAG-U.Sao Paulo <antonio@astro.iag.usp.br>
A.M. Magalh�es, A. Pereyra, T. Dominici and Z. Abraham (IAG, U.
of S�o Paulo) report:
We have obtained R band imaging polarimetry of GRB030329. The
observation was performed with the IAGPOL imaging polarimeter and the
IAG-USP 60cm telescope, at the Laborat�rio Nacional de Astrof�sica site.
The observation spanned the period UT 01:06 to 01:51 on 31 March,
1.519 - 1.592 days after the event, and was made through less than ideal
sky conditions. The double-beam technique provided by a calcite plate
analyser ensured however that useful polarimetric data could still be
obtained.
We have measured the following linear polarization for GRB030329:
P = [1.974 +- 0.477] %, position angle = 83.2 degrees.
(The position angle is already in the equatorial system, increasing from
North to East.)
As a comparison, we have obtained much smaller polarization for field
objects. In particular, for USNO A-2.0 1050-6351075 (object 'A' in
Rumyantsev et al, GCN 2005) and 1050-6349885, we have obtained the
following linear polarization values: [0.58 +- 0.11] at 66deg and [0.06
+- 0.12] at 21deg, respectively. This low foreground polarization is
consistent with the very low foreground reddening towards the field
(E(B-V)=0.025, Schlegel et al. 1998).
Dilution of the GRB intrinsic polarization by the much fainter (Blake &
Bloom GCN 2011) host galaxy should be negligible. Polarization within
the host galaxy interstellar medium seems unlikely due to the small
internal extincion (Caldwell et al. GCN 2053); this should be settled
by variability studies and spectropolarimetry.
We conclude that a substantial fraction of the GRB030329 polarization
is intrisic in nature. This strongly suggests that GRB030329 is
non-spherically symmetric, lending further credence to the asymmetric
fireball scenario (in combination with a physical process that produces
polarization, such as synchtron radiation). The position angle above
should be correlated with the object's beam or jet orientation on the
sky. It would be interesting to see whether a similar position angle
is present in forthcoming resolved radio observations of the source.
This message may be cited.
[GCN OPS NOTE (22Apr03): The circular was originally submitted
on 14 Apr 03, but was delayed due to email address mismatch.]