{
  "bibcode": "2022GCN.32995....1L",
  "body": "E. Lindfors (FINCA), K. Nilsson (FINCA), I. Liodakis (FINCA), A. Kasikov \n(NOT, Aarhus University, Tartu Observatory), I. Negueruela (University \nof Alicante)\n\nWe observed gamma-ray burst GRB221009A following the GCN alert #32632 \n(Dichiara et al., 2022) in optical polarization. The source was observed \nwith the Nordic Optical Telescope (NOT) and the ALFOSC instrument in the \nR-band using the standard setup for linear polarization observations \n(lambda/2 retarder followed by a calcite). The observations started \n2022-10-12 at 20:15UT, i.e. approximately 3 days and 6 hours after the \ntrigger (2022-10-09 14:10:17 UT). The observations were performed with \nclear sky with seeing 1.2 arcseconds. As the GRB occurred in crowded \ngalactic field the ordinary beam image of the GRB221009A ended up behind \nthe extraordinary beam of the nearby bright star. Therefore, we had to \nperform careful modelling of the PSF using the second bright star in the \nfield of view and subtracted the modeled PSF from the image. We repeated \nthis for each image separately. As the PSF subtraction can result in \nsome artifacts to the background, we measured the resulting images with \nsmall aperture of 1.5 arcsec radius. Using the standard formulas, we \nderived a 2 sigma upper limit of 5.1% percent on the polarization degree.",
  "circularId": 32995,
  "createdOn": 1669848503000,
  "email": "jliodakis@gmail.com",
  "subject": "Optical polarization observation of GRB 221009A",
  "submitter": "Ioannis Liodakis at Finnish Centre for Astronomy with ESO  <jliodakis@gmail.com>",
  "eventId": "GRB 221009A"
}