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The black hole in the binary system Swift J1727.8-1613 flickers in X-rays like a campfire in the wind. But if you look closely at the rhythm of this flickering, you can see distinct 'humps'—bursts at specific frequencies, like drumbeats in the noise of a crowd. Astronomers caught two such humps in data from the Chinese telescope Insight-HXMT, which observed the system in the hard X-ray range up to 100 keV.

It turned out that the beat frequency increases with photon energy: the harder the light, the faster the pulse. But only up to 30 keV — beyond that, the frequency stalls, as if the drummer got tired of speeding up. Meanwhile, the amplitude of the oscillations (how much the flux changes) keeps growing and reaches 15% in the hardest range of 50–100 keV. This means the source of the pulsations sits in a very hot corona — a cloud of scorching particles hovering right above the black hole, like steam over a boiling pot.

When the astronomers applied the model of relativistic precession (the orbit of matter wobbles slightly, like a spinning top) they could estimate the black hole's mass and spin. From the full dataset, the mass came out between 2.84 and 120 solar masses, and the spin parameter between 0.14 and 0.43. If you take only the hardest bands, where the frequency is stable, the constraints tighten: mass no more than 14 solar masses, and spin no more than 0.40. That's a very modest spin — the black hole is far from spinning at its limit.

Such measurements help us understand how matter falls into a black hole and why the X-ray corona behaves the way it does. Each 'hump' in the spectrum is like a fingerprint left by matter on its last orbits before plunging into the abyss.