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 ν and the occurrence of X-ray bursts: when ν ≳ 9 mHz, no bursts occur, whereas ν ≲ 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 ν 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.