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We investigated the convexity of all type I X-ray bursts with millihertzquasi-periodic oscillations (mHz QPOs) in 4U 1636-53 using archivalobservations with the Rossi X-ray Timing Explorer. We found that, at a 3.5$\sigma$ confidence level, in all 39 cases in which the mHz QPOs disappeared atthe time of an X-ray burst, the convexity of the burst is positive. Theconvexity measures the shape of the rising part of the burst light curve and,according to recent models, it is related to the ignition site of bursts on theneutron star surface. This finding suggests that in 4U 1636-53 these 39 burstsand the marginally-stable nuclear burning process responsible for the mHz QPOstake place at the neutron-star equator. This scenario could explain theinconsistency between the high accretion rate required for triggering mHz QPOsin theoretical models and the relatively low accretion rate derived fromobservations.

Millihertz quasi-periodic oscillations in 4U 1636-53 associated with bursts with positive convexity only