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

Abstract

We investigated the convexity of all type I X-ray bursts with millihertz\nquasi-periodic oscillations (mHz QPOs) in 4U 1636-53 using archival\nobservations with the Rossi X-ray Timing Explorer. We found that, at a 3.5\n$\\sigma$ confidence level, in all 39 cases in which the mHz QPOs disappeared at\nthe time of an X-ray burst, the convexity of the burst is positive. The\nconvexity measures the shape of the rising part of the burst light curve and,\naccording to recent models, it is related to the ignition site of bursts on the\nneutron star surface. This finding suggests that in 4U 1636-53 these 39 bursts\nand the marginally-stable nuclear burning process responsible for the mHz QPOs\ntake place at the neutron-star equator. This scenario could explain the\ninconsistency between the high accretion rate required for triggering mHz QPOs\nin theoretical models and the relatively low accretion rate derived from\nobservations.\n