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Burst oscillations are brightness asymmetries that develop in the burningocean during thermonuclear bursts on accreting neutron stars. They have beenobserved during H/He-triggered (Type I) bursts and Carbon-triggeredsuperbursts. The mechanism responsible is not unknown, but the dominant burstoscillation frequency is typically within a few Hz of the spin frequency, wherethis is independently known. One of the best-studied burst oscillation sources,4U 1636-536, has oscillations at $581\,\text{Hz}$ in both its regular Type Ibursts and in one superburst. Recently however, Strohmayer \& Mahmoodifarreported the discovery of an additional signal at a higher frequency,$835\,\text{Hz}$, during the superburst. This higher frequency is consistentwith the predictions for several types of global ocean mode, one of thepossible burst oscillation mechanisms. If this is the case then the samephysical mechanism may operate in the normal Type I bursts of this source. Inthis paper we report a stacked search for periodic signals in the regular TypeI bursts: we found no significant signal at the higher frequency, with upperlimits for the single trial root mean square (rms) fractional amplitude of0.57(6)\%. Our analysis did however reveal that the dominant $581\,\text{Hz}$burst oscillation signal is present at a weak level even in the sample ofbursts where it cannot be detected in individual bursts. This indicates thatany cutoff in the burst oscillation mechanism occurs below the detectionthreshold of existing X-ray telescopes.

A search for the $835\,\text{Hz}$ superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536