Millihertz oscillation frequency drift predicts the occurrence of type I X-ray bursts

The millihertz quasi-periodic oscillations (mHz QPOs) discovered 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. We show that, close to the transition between the island and the banana state, 4U 1636 - 53 exhibits mHz QPOs whose frequencies systematically decrease with time, until the oscillations disappear and a type I X-ray burst occurs. There is a strong correlation between the QPO frequency v and the occurrence of X-ray bursts: when v greater than or similar to 9 mHz, no bursts occur, whereas v less than or similar to 9 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 kiloseconds after v drops below 9 mHz. This observational result confirms that the mHz QPO phenomenon is intimately related to the processes that lead to a thermonuclear burst.