The IXPE View of GRB 221009A
Authors: Negro Michela , Di Lalla Niccolò , Omodei Nicola , Veres Péter , Silvestri Stefano , Manfreda Alberto , Burns Eric , Baldini Luca , Costa Enrico , Ehlert Steven R. , Kennea Jamie A. , Liodakis Ioannis , Marshall Herman L. , Mereghetti Sandro , Middei Riccardo , Muleri Fabio , O’Dell Stephen L. , Roberts Oliver J. , Romani Roger W. , Sgró Carmelo , Terashima Masanobu , Tiengo Andrea , Viscolo Domenico , Di Marco Alessandro , La Monaca Fabio , Latronico Luca , Matt Giorgio , Perri Matteo , Puccetti Simonetta , Poutanen Juri , Ratheesh Ajay , Rogantini Daniele , Slane Patrick , Soffitta Paolo , Lindfors Elina , Nilsson Kari , Kasikov Anni , Marscher Alan P. , Tavecchio Fabrizio , Cibrario Nicoló , Gunji Shuichi , Malacaria Christian , Paggi Alessandro , Yang Yi-Jung , Zane Silvia , Weisskopf Martin C. , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Donnarumma Immacolata , Doroshenko Victor , Dovc̆iak Michal , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Garcia Javier A. , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Kaaret Philip , Karas Vladimir , Kislat Fabian , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , Maldera Simone , Marin Frédéric , Marinucci Andrea , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Ng C.-Y. , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Ramsey Brian D. , Rankin John , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Wu Kinwah , Xie Fei 
Published: March 2023

We present the IXPE observation of GRB 221009A, which includes upper limits on the linear polarization degree of both prompt and afterglow emission in the soft X-ray energy band. GRB 221009A is an exceptionally bright gamma-ray burst (GRB) that reached Earth on 2022 October 9 after traveling through the dust of the Milky Way. The Imaging X-ray Polarimetry Explorer (IXPE) pointed at GRB 221009A on October 11 to observe, for the first time, the 2–8 keV X-ray polarization of a GRB afterglow. We set an upper limit to the polarization degree of the afterglow emission of 13.8% at a 99% confidence level. This result provides constraints on the jet opening angle and the viewing angle of the GRB, or alternatively, other properties of the emission region. Additionally, IXPE captured halo-rings of dust-scattered photons that are echoes of the GRB prompt emission. The 99% confidence level upper limit to the prompt polarization degree depends on the background model assumption, and it ranges between ∼55% and ∼82%. This single IXPE pointing provides both the first assessment of X-ray polarization of a GRB afterglow and the first GRB study with polarization observations of both the prompt and afterglow phases.