Probing the interstellar medium toward GRB 221009A through X-ray dust scattering

¹ Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, 27100 Pavia, Italy
² Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo, (TN), Italy
³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
⁴ Ruder Bošković Institute, Bijenič ka cesta 54, 10000 Zagreb, Croatia
⁵ University of Zagreb Faculty of Electrical Engineering and Computing, Unska ul. 3, 10000 Zagreb, Croatia
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, F-75005 Paris, France
⁸ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, (LC), Italy
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

^⋆ Corresponding author; beatrice.vaia@iusspavaia.it

Received: 25 November 2024
Accepted: 28 February 2025

Abstract

The observation of 21 X-ray dust-scattering rings around the extraordinarily bright gamma-ray burst (GRB) 221009A provides a unique opportunity to study the interstellar medium (ISM) through which the X-ray radiation traveled in our Galaxy and, by extension, in the host galaxy as well. In particular, since the ring intensity and radius at a given time depend on the amount of dust and on its distance, respectively, XMM-Newton and Swift images allowed us to map the ISM around the direction of the GRB with better resolution than in the existing optical- and infrared-based 3D dust maps, both in the plane of the sky (a few arcminutes) and along the line of sight (from ≃1 pc for dust clouds within 1 kpc to ≃100 pc for structures at distances larger than 10 kpc). As a consequence, we could revise prior estimates of the GRB soft X-ray fluence, obtaining a ∼35% lower value, which, however, still indicates a substantial excess with respect to the extrapolation of the spectral models constrained by hard X-ray observations. Additionally, we detect significant spectral variability in two azimuthal sectors of the X-ray rings, which can be fully attributed to different Galactic absorption in these two directions. The comparison of the total hydrogen column density inferred from spectral fitting, with the Galactic contribution derived from the intensity of the X-ray rings, in the same sectors allowed us to more robustly constrain the absorption in the host galaxy to N_{H, z = 0.151} = (3.7 ± 0.3)×10²¹ cm⁻². This result is relevant not only for characterizing the ISM of the host galaxy and understanding how the GRB radiation might have affected it, but also for modeling the broadband spectrum of the GRB afterglow and constraining the properties of a possible underlying supernova.