  GRB 221009A is the brightest gamma-ray burst (GRB) ever detected and occurred
at low Galactic latitude. Owing to this exceptional combination, its prompt
X-ray emission could be detected for weeks in the form of expanding X-ray rings
produced by scattering in Galactic dust clouds. We report on the analysis of 20
rings, generated by dust at distances ranging from 0.3 to 18.6 kpc, detected
during two XMM-Newton observations performed about 2 and 5 days after the GRB.
By fitting the spectra of the rings with different models for the dust
composition and grain size distribution, we reconstructed the spectrum of the
GRB prompt emission in the 0.7-4 keV energy range as an absorbed power law with
photon index 1-1.4 and absorption in the host galaxy nHz=(4.1-5.3)E21 cm-2.
Taking into account the systematic uncertainties on the column density of dust
contained in the clouds producing the rings, the 0.5-5 keV fluence of GRB
221009A can be constrained between 1E-3 and 7E-3 erg cm-2.
