Relativistic ocean $r$-modes during type-I X-ray bursts

Abstract

Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. These frequencies can be offset from the NS spin frequency by several Hz (where known independently) and can drift by 1-3 Hz. One plausible explanation is that a wave is present in the bursting ocean, the rotating frame frequency of which is the offset. The frequency of the wave should decrease (in the rotating frame) as the burst cools hence explaining the drift. A strong candidate is a buoyant $r$-mode. To date, models that calculated the frequency of this mode taking into account the radial structure neglected relativistic effects and predicted rotating frame frequencies of $\sim$ 4 Hz and frequency drifts of > 5 Hz; too large to be consistent with observations. We present a calculation that includes frame-dragging and gravitational redshift that reduces the rotating frame frequency by up to 30 % and frequency drift by up to 20 %. Updating previous models for the ocean cooling in the aftermath of the burst to a model more representative of detailed calculations of thermonuclear X-ray bursts reduces the frequency of the mode still further. This model, combined with relativistic effects, can reduce the rotating frequency of the mode to $\sim$ 2 Hz and frequency drift to $\sim$ 2 Hz, which is closer to the observed values.

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Cited by in corpus (11)

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• Nuclear-powered X-ray millisecond pulsars
• Pulse Profile Modeling of Thermonuclear Burst Oscillations I: The Effect of Neglecting Variability
• Premerger phenomena in neutron-star binary coalescences
• Pulse Profile Modelling of Thermonuclear Burst Oscillations II: Handling variability
• Multi-messenger Emission from Tidal Waves in Neutron Star Oceans
• Waves in Thin Oceans on Oblate Neutron Stars
• Gravitational waves from r-mode oscillations of stochastically accreting neutron stars
• Impact of Accretion Assumptions on Pulse Profile Modelling of Superburst Oscillations in 4U 1636-536
• Modelling Neutron-Star Ocean Dynamics
• An ultraviolet burst oscillation candidate from the low-mass X-ray binary EXO 0748-676