TITLE:   GCN CIRCULAR
NUMBER:  32995
SUBJECT: Optical polarization observation of GRB 221009A
DATE:    22/11/30 22:48:23 GMT
FROM:    Ioannis Liodakis at Finnish Centre for Astronomy with ESO  <jliodakis@gmail.com>

E. Lindfors (FINCA), K. Nilsson (FINCA), I. Liodakis (FINCA), A. Kasikov 
(NOT, Aarhus University, Tartu Observatory), I. Negueruela (University 
of Alicante)

We observed gamma-ray burst GRB221009A following the GCN alert #32632 
(Dichiara et al., 2022) in optical polarization. The source was observed 
with the Nordic Optical Telescope (NOT) and the ALFOSC instrument in the 
R-band using the standard setup for linear polarization observations 
(lambda/2 retarder followed by a calcite). The observations started 
2022-10-12 at 20:15UT, i.e. approximately 3 days and 6 hours after the 
trigger (2022-10-09 14:10:17 UT). The observations were performed with 
clear sky with seeing 1.2 arcseconds. As the GRB occurred in crowded 
galactic field the ordinary beam image of the GRB221009A ended up behind 
the extraordinary beam of the nearby bright star. Therefore, we had to 
perform careful modelling of the PSF using the second bright star in the 
field of view and subtracted the modeled PSF from the image. We repeated 
this for each image separately. As the PSF subtraction can result in 
some artifacts to the background, we measured the resulting images with 
small aperture of 1.5 arcsec radius. Using the standard formulas, we 
derived a 2 sigma upper limit of 5.1% percent on the polarization degree.