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

## TL;DR

This study searched for a hypothesized 835 Hz oscillation in regular bursts of 4U 1636-536 but found no significant signal, suggesting the mechanism may be below current detection limits, while confirming the persistent 581 Hz oscillation.

## Contribution

The paper provides the first systematic search for the 835 Hz signal in regular bursts and sets upper limits, refining understanding of burst oscillation mechanisms.

## Key findings

- No significant 835 Hz signal detected in regular bursts.
- The 581 Hz oscillation is weakly present even when not individually detectable.
- Upper limits on fractional amplitude at 835 Hz are established.

## Abstract

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 Hz of the spin frequency, where this 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 I bursts and in one superburst. Recently however, Strohmayer \& Mahmoodifar reported the discovery of an additional signal at a higher frequency, $835\,\text{Hz}$, during the superburst. This higher frequency is consistent with the predictions for several types of global ocean mode, 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 (rms) fractional amplitude of 0.57(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 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.