Mapping our own galaxy is trickier than it sounds. The Solar System sits well embedded inside the disc of the Milky Way, so there's no way to step back and see its structure from above, the way we do with other spiral galaxies. Thick clouds of cosmic dust make things harder still, hiding entire regions from view, especially in the far outer arms.
A team led by Beatrice Vaia of the Istituto Nazionale di Astrofisica (INAF), Italy, found a way around these obstacles. Instead of studying stars directly, the researchers used X-ray light from three explosions in distant galaxies, tracking how it scattered off dust within the Milky Way itself.
How X-rays trace rings in dust
Gamma-ray bursts (GRBs) are among the most powerful explosions in the Universe, occurring in far-off galaxies and releasing intense bursts of X-ray light. As that light travels through the Milky Way on its way to Earth, some of it scatters off dust grains sitting in our galaxy's spiral arms.
The scattered light reaches telescopes slightly delayed compared to the direct beam, forming a bright ring on the sky around the point of the explosion. Over time, this ring slowly expands.
The rate of that expansion depends on the distance to the dust cloud doing the scattering. By measuring it, the team could work out precise distances to the dust clouds, and therefore to the galactic arms in which those clouds sit. The method stays accurate even at large distances, where other techniques become less reliable.
Outer arms turn out to be farther than thought
The researchers combined observations of three gamma-ray bursts: GRB 221009A (2022), GRB 160623A (2016), and GRB 031203 (2003). Their dust-scattered echoes were picked up by XMM-Newton and Chandra between December 2003 and November 2022.
The results confirmed the previously known distance to the Perseus arm. But two other arms — the Outer Scutum-Centaurus Arm and the Outer Arm — turned out to lie up to 10% farther away than earlier models suggested.
Before this method, distances to the outer arms were mostly modelled indirectly, based on the Galaxy's rotation. That approach left room for error. Direct measurement through X-ray echoes has narrowed that gap.
Chandra, XMM-Newton and Gaia complement each other
ESA's Gaia telescope has transformed our picture of the Milky Way in recent years, its data helping establish that the galaxy has four spiral arms rather than the two once assumed. But for the most distant parts of the disc, Gaia's distance measurements are less precise.
That is where the X-ray method proves especially valuable, staying accurate at distances where optical measurements start to lose their edge.
Now in its third decade, XMM-Newton continues to return a steady stream of groundbreaking science on everything from the brightest-ever GRB, to stars being shredded by black holes, to X-ray snapshots of Mars. It's even more exciting when missions team up, as they did here. Together, they can reveal huge amounts about the skies around us.Erik Kuulkers, ESA XMM-Newton project scientist
Fainter echoes and fresh Gaia data ahead
The study, published on 29 June in Astronomy & Astrophysics, is not the final word. The map of the Milky Way will keep sharpening in the coming years as new Gaia data arrives: the mission's fourth data release is planned for December 2026, with a fifth expected sometime after 2030.
Meanwhile, the next generation of X-ray astronomy is on its way. ESA's NewAthena observatory is expected to detect far fainter X-ray echoes than XMM-Newton and Chandra can, opening up even more distant and dimmer regions of the outer Galaxy to study.
