Astrophysicists have revealed a way of studying the extreme environments near SMBHs by identifying a unique pattern of flickering light that may disclose how gas and plasma fall toward these cosmic giants, which are billions to trillions of times more massive than our Sun. Though well-known for swallowing everything that comes close to them, black holes can also flicker and pulse with amazing regularity. This flickering of observed electromagnetic radiations (lights of all wavelengths) is called quasi-periodic oscillations (QPOs)—a pattern of brightening and dimming seen at regular intervals in the light emitted from the extremely hot matter falling onto a black hole known as accretion flow. QPOs occur at specific frequencies and provide a unique way to probe the physics of black holes, much like how seismologists use earthquakes to study Earth’s interior.

QPOs are one of the direct clues we have for studying the black holes. Scientists can infer the properties of spacetime near the event horizon, which is known as the point of no return (even light can escape from it). These oscillations can tell us about the dynamics of accretion, the spin of the black hole, and even test Einstein’s theory of general relativity under extreme conditions. Until now, QPOs observed in recent years from SMBH sources have mostly appeared at milliHertz (mHz) frequencies, meaning the flickering occurs over timescales of minutes to hours. However, this study predicts QPOs at much higher cHz (centi-Hertz) frequencies and beyond, which occur at timescales 10 to 100 times shorter than expected before. This is crucial because it suggests a new, previously debated type of matter flow known as low-angular momentum accretion and its influences on the black hole’s feeding process to make it detectable.

Using cutting-edge general relativistic magnetohydrodynamic (GRMHD) simulations, the research team modeled how matter with low angular momentum falls onto SMBHs. This study shows the formation and the role of the pseudo surface (PS)—a region near the black hole where low-angular momentum matter accumulates before plunging inward. Unlike the event horizon of a traditional black hole, which acts as a one-way boundary where nothing can escape, the PS forms due to the slow radial infall of gas that temporarily piles up before falling in. This region significantly alters the accretion dynamics, creating a feedback mechanism where matter interacts with itself before crossing the event horizon. The interaction at this PS leads to periodic fluctuations in density and pressure, which in turn generate resonant oscillations that appear as cHz QPOs in the light emitted. For the first time, these simulations revealed that such flows can naturally produce cHz QPOs—flickering at much higher frequencies than previously expected. These oscillations occur in a 2:1 harmonic ratio, meaning that if one flicker happens at a given frequency, another follows at exactly double the rate. By linking this with recent observations of Sagittarius A* (the supermassive black hole at our galaxy center) and other active black holes, the study suggests that detecting these high-frequency QPOs in real observations could provide direct evidence of low-angular momentum accretion flows—a missing piece in black hole physics.