Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts

Millihertz quasi-periodic oscillations reported in three neutron-star low mass X-ray binaries have been suggested to be a mode of marginally stable nuclear burning on the neutron star surface. In this Letter, we show that close to the transition between the island and the banana state, 4U~1636--53 shows mHz QPOs whose frequency systematically decreases with time until the oscillations disappear and a Type I X-ray burst occurs. There is a strong correlation between the QPO frequency $\nu$ and the occurrence of X-ray bursts: when $\nu\gtrsim9$ mHz no bursts occur, while $\nu\lesssim9$ mHz does allow the occurrence of bursts. The mHz QPO frequency constitutes the first identified observable that can be used to predict the occurrence of X-ray bursts. If a systematic frequency drift occurs, then a burst happens within a few kilo-seconds after $\nu$ drops below 9 mHz. This observational result confirms that the mHz QPO phenomenon is intimately related with the processes that lead to a thermonuclear burst.