  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 travelling 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 which 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% to ~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.
