For thirteen straight years, the Swift space observatory recorded the X-ray "breathing" of the pulsar GX 301-2. Every few minutes, the BAT instrument measured how much hard radiation came from this pair — a neutron star and its massive companion. When you overlay all the recordings on one plot, you don't see a flat line but waves: the brightness rises and falls.

In these waves, astronomers found two rhythms at once. The main one is 115 days: in that time, the star manages to flare up and fade again. The second, weaker one, is 65 days. It's not independent: it's like two musicians playing at different tempos, and in the hall you hear not only their notes but also a slow "beat" from their difference. Same here: 65 days is the difference between 115 days and the orbital period of the pair — the time it takes the neutron star to make one revolution around its neighbor.

Where does the 115-day rhythm come from? Most likely, the stellar wind — the stream of gas flowing from the giant's surface — contains clumps. They rotate with the star, like spokes in a wheel. When such a clump passes near the neutron star, the star captures more gas and flares brighter. After 115 days, the next "spoke" — and another burst. This mechanism also explains why periods in different systems line up: the larger the orbit, the slower the wind wheel turns.

GX 301-2 turned out to be a rare case where both rhythms are visible — the main one and the beat. It's like hearing not only the main melody in the orchestra's noise but also a quiet accompaniment. Further observations will show how true this picture is for other wind systems.