Abstract

GRB 221009A is the brightest gamma-ray burst (GRB) ever detected that has 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 X−ray Multi Mirror (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 Γ = 1-1.4 and absorption in the host galaxy N _H,z = (4.1-5.3) × 10²¹ cm^-2. Taking into account the systematic uncertainties regarding 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 10^-3 and 7 × 10^-3 erg cm^-2. Both the fluence and the photon index indicate the presence of a possible soft excess with respect to the extrapolation of the main GRB peak observed at higher energies.