Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts

TL;DR

This paper models oceanic r-modes excited during superbursts in accreting neutron stars, explaining observed oscillation frequencies and drifts, and compares predictions with observations to understand burst oscillation mechanisms.

Contribution

It provides the first detailed calculation of oceanic r-mode frequency evolution during superbursts and compares these predictions with observational data.

Findings

Mode frequencies during superbursts range from 4-14 Hz

Predicted frequencies imply a neutron star spin of about 592 Hz

Model overestimates frequency drift by approximately 90%

Abstract

Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The frequencies can be offset from the spin frequency of the NS by several Hz, and can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean, the frequency of which would decrease (in the rotating frame) as the burst cools, hence explaining the drifts. Most burst oscillations have been observed during H/He triggered bursts, however there has been one observation of oscillations during a superburst; hours' long Type I X-ray bursts caused by unstable carbon burning deeper in the ocean. This paper calculates the frequency evolution of an oceanic r-mode during a superburst. The rotating frame frequency varies during the burst from 4-14 Hz, and is sensitive to the background parameters, in particular the temperature of the ocean and ignition depth. This calculation is compared to the superburst oscillations observed on 4U-1636-536. The predicted mode frequencies ($\sim$ 10 Hz) would require a spin frequency of $\sim$ 592 Hz to match observations; 6 Hz higher than the spin inferred from an oceanic r-mode model for the H/He triggered burst oscillations. This model also over-predicts the frequency drift during the superburst by 90 %.