In 2018, something strange happened to the galaxy 1ES 1927+654: its X-ray 'corona'—a cloud of scorching particles around the supermassive black hole — vanished. Over the next few years, it slowly came back. A team of astronomers decided not just to watch the brightness but to listen to the 'tremor' of that glow.
They pulled data from the XMM-Newton space observatory for 2019–2024 and applied a method similar to analyzing noise in music. Instead of notes, they had random X-ray flares, which they described with different mathematical models (Gaussian processes), comparing which fit the observations best. And they found something surprising: on May 5, 2019, the character of that 'tremor' changed abruptly right in the middle of one continuous observing session.
At first, the variability resembled slow, smooth waves with a long memory of past flares. Then it became more jittery, with a short memory, as if the system 'forgot' its previous states faster. This happened before the corona became noticeably brighter and harder, i.e., before the external signs of its recovery appeared. Later, in 2022–2024, when the object developed a jet (a stream of matter) and periodic oscillations, the tremor's character changed again — a faster, more orderly pulsation emerged. Such fine-grained analysis of random light variations lets us peer into the hidden mechanics of the black hole's surroundings as they restructure.
The change in the 'tremor' character took just a few hours — faster than the corona's brightness itself changes noticeably.
In Carl Sagan's novel 'Contact,' a signal from space is first mistaken for random noise — until a hidden rhythm is found in it.