A flashlight shines through fog, and its light changes color slightly. From that change, you can tell what the fog is made of. That's how astronomers study cold gas around X-ray sources: iron atoms in the gas absorb and re-emit X-rays at a very specific energy, like a fingerprint. But there's a catch: if you strip a few electrons from the atom, the line shifts. The new Japanese telescope XRISM can detect shifts tens of times smaller than what was possible before.
In the Centaurus X-3 system, a neutron star (a tiny, superdense remnant of an exploded star) orbits a massive ordinary star and pulls material from it. Near the neutron star, there's a cloud of cold gas that glows with iron lines. Scientists compared two lines — one at 6.4 keV, the other at 7.06 keV, and saw they were shifted differently. If the gas were just moving as a whole, both lines would shift the same way. The difference indicates that the iron atoms have lost about five of their twenty-six electrons. It's like if someone's jacket lost its buttons — the jacket is the same, but it fastens differently.
Knowing how many electrons were stripped, astronomers calculated how much of the line shift was due to that, rather than to the gas's motion. The residual shift matched optical measurements of the system's overall motion. This allowed them to refine where the cold cloud is located — closer to or farther from the neutron star than previously thought. The same trick can be applied to any other source where iron glows, from black holes to supernova remnants. The flashlight has gotten better at distinguishing the fog.
The energy of an X-ray photon at 6.4 keV is a billion times greater than that of visible light, yet XRISM measures its shift with an accuracy of 0.2 electronvolts: that's like noticing a person in a crowd has moved by the width of a hair.
In the series 'The Expanse', ships are tracked by their heat radiation, but here, cold gas is revealed by iron's X-ray lines, almost like a lie detector for cosmic dust.