Accurate distances of the Galactic spiral arms from dust-scattered X-ray emission of gamma-ray bursts
The details of the spiral structure of the Milky Way are still debated due to large uncertainties in the distance estimates obtained through the most common tracers. X-ray dust scattering rings produced by short extragalactic X-ray transients provide instead a direct method to measure the 3D distribution of interstellar clouds up to the edges of our Galaxy with a few percent precision. We report on the analysis of all the available XMM-Newton and Chandra follow-up observations of three low-latitude gamma-ray bursts: GRB 031203 ($l \sim 255{\deg}$, $b \sim -5{\deg}$), GRB 160623A ($l \sim 84{\deg}$, $b \sim -3{\deg}$), and GRB 221009A ($l \sim 53{\deg}$, $b \sim 4{\deg}$). The previous detection of X-ray rings in these observations, produced by dust clouds located beyond 5 kpc, can be associated with dust in the Perseus, Outer, and Outer Scutum-Centaurus arms, thus providing direct distance measurements to these structures along three distinct lines of sight. We have identified two additional rings in the direction of GRB 160623A, produced by dusty clouds at $6.91\, \pm\,0.06$ kpc and $9.9\,\pm\,0.6$ kpc, and confirmed -- through a second XMM-Newton observation -- the presence of one cloud at $9.7\,\pm\,0.4$ kpc toward GRB 031203. We also accurately measured the distance of dusty clouds up to $19.0\,\pm\,0.2$ kpc owing to the analysis of one Chandra and four XMM-Newton observations of GRB 221009A. The small statistical and systematic uncertainties of these measurements place tight constraints on the geometry of the outer Milky Way and reveal significant deviations from current models, which critically depend on spectroscopy-based Galactic rotation curves at large distances.