Dust Rings Reveal the Secrets of the Spiral Arms express
Imagine a dark room with dust floating in the air. A bright flashlight goes off, and the dust particles light up, revealing their positions. Astronomers have applied a similar trick to our Galaxy. The flashlights were gamma-ray bursts, the most powerful explosions in distant galaxies, and the dust particles were interstellar clouds. X-rays from a burst pass through such a cloud, some of the radiation is deflected, and we see a glowing ring. The delay between the burst and the ring's appearance depends on the distance to the cloud, and it can be measured to within a few percent.
In the new study, the team analyzed observations of three gamma-ray bursts made with the XMM-Newton and Chandra telescopes. The bursts illuminated dust clouds in the directions of the Perseus arm, the Outer arm, and the Outer Scutum-Centaurus arm. They not only confirmed distances to known clouds but also found two new rings. These lie at about 7 and 10 kiloparsecs (23,000 and 33,000 light-years) in the direction of one burst. Analysis of multiple observations of the brightest one, GRB 221009A, allowed them to measure the distance to a cloud at 19 kiloparsecs (62,000 light-years) with an uncertainty of just 0.2 kiloparsecs.
Such precise measurements show that the actual geometry of the Milky Way's outer arms differs noticeably from what current models predict. Those models rely on spectroscopic measurements of the Galaxy's rotation speeds. It seems our stellar home is a bit more complex than we thought. Dust rings, like echoes of distant explosions, help us probe structures invisible to the eye and refine the map of the Galaxy. A flashlight in a dark room revealed the outlines of dust, and so gamma-ray bursts are gradually bringing the pattern of galactic arms into focus.
The dust clouds that gamma-ray bursts illuminate are made mostly of tiny particles about the size of cigarette smoke — less than a micron across. But there are so many of them that together they scatter X-rays noticeably.
In Arthur C. Clarke's novel "Light of Other Days," people learn to see through any barrier using wormholes. Here, nature itself has given us a way to peer through the Galaxy's dust.