2019ApJ...876...11V

Burst oscillations are brightness asymmetries that develop in the burning ocean during thermonuclear bursts on accreting neutron stars. They have been observed during H/He-triggered (Type I) bursts and carbon-triggered superbursts. The mechanism responsible is not unknown, but the dominant burst oscillation frequency is typically within a few hertz of the spin frequency, where this is independently known. One of the best-studied burst oscillation sources, 4U 1636-536, has oscillations at 581 Hz in both its regular Type I bursts and in one superburst. Recently, however, Strohmayer & Mahmoodifar reported the discovery of an additional signal at a higher frequency, 835 Hz, during the superburst. This higher frequency is consistent with the predictions for several types of global ocean modes, one of the possible burst oscillation mechanisms. If this is the case then the same physical mechanism may operate in the normal Type I bursts of this source. In this paper we report a stacked search for periodic signals in the regular Type I bursts: we found no significant signal at the higher frequency, with upper limits for the single trial root-mean-square fractional amplitude of 0.57(6)%. Our analysis did, however, reveal that the dominant 581 Hz burst oscillation signal is present at a weak level even in the sample of bursts where it cannot be detected in individual bursts. This indicates that any cutoff in the burst oscillation mechanism occurs below the detection threshold of existing X-ray telescopes.