X-ray echoes revise the distance to outer arms

ESA’s XMM-Newton and NASA’s Chandra space telescopes caught the echoes of three distant explosions — gamma-ray bursts (GRBs) in far-off galaxies. Their X-ray light passed through the Milky Way and scattered off dust grains in the spiral arms. This formed bright rings that the telescopes picked up.

The rings slowly expanded. From the rate of that expansion, the team led by Beatrice Vaia of INAF (Italy) worked out the distance to the dust clouds, and therefore to the arms themselves. The method stays accurate even at large distances.

The result: the distance to the Perseus arm was confirmed, while two others — the Outer Scutum-Centaurus Arm and the Outer Arm — turned out to be up to 10% farther. The data rest on GRBs 221009A (2022), 160623A (2016) and 031203 (2003), observed between 2003 and 2022.

The outer Galaxy is hard to study: the Solar System sits inside the disc, and much of it is hidden by dust. The arms used to be modelled indirectly — through the Galaxy’s rotation, which left room for error. The Gaia telescope gives precise stellar distances, but its measurements are less reliable for the outer arms.

Fainter echoes will be the target of ESA’s next X-ray observatory, NewAthena.