A lighthouse that changes the color of each flash in time with its rotation: that's one way to describe what astronomers saw when they pointed the latest X-ray telescope, XRISM, at the pulsar Hercules X-1. This object is a neutron star pulling matter from a companion ordinary star. As it falls, the matter spirals into narrow columns along magnetic field lines and slams into the poles at enormous speeds — tens of thousands of kilometers per second.

In such a furnace, iron atoms caught in the flow emit a characteristic X-ray glow. Previously, this appeared as a smeared line, like a note blurred into a chord by frantic motion. Now, the Resolve instrument aboard XRISM, working like a microphone with perfect pitch, has resolved the fine structure of this signal. It turns out the line's brightness varies with the pulsar's phase: when one of the hot pillars faces us, the line flares up; when it turns away, it fades.

But the most intriguing discovery lies in the long 35-day modulation. The pulsar seems to rock slowly, and its accretion columns rock along with it. This dance, the neutron star's precession, affects how we see the iron line's pulsations. Watching this dance is like listening to the changing pitch of a passing siren: from the shift in tone, you can deduce its motion. Similarly, by tracking variations in the iron line, scientists hope to measure how the neutron star rotates inside — something that remains a mystery.