The accretion rate dependence of burst oscillation amplitude

Abstract

Neutron stars in low mass X-ray binaries exhibit oscillations during thermonuclear bursts, attributed to asymmetric brightness patterns on the burning surfaces. All models that have been proposed to explain the origin of these asymmetries (spreading hotspots, surface waves, and cooling wakes) depend on the accretion rate. By analysis of archival RXTE data of six oscillation sources, we investigate the accretion rate dependence of the amplitude of burst oscillations. This more than doubles the size of the sample analysed previously by Muno et al. (2004), who found indications for a relationship between accretion rate and oscillation amplitudes. We find that burst oscillation signals can be detected at all observed accretion rates. Moreover, oscillations at low accretion rates are found to have relatively small amplitudes ($A_{\text{rms}}\le 0.10$) while oscillations detected in bursts observed at high accretion rates cover a broad spread in amplitudes ($0.05\le A_{\text{rms}}\le 0.20$). In this paper we present the results of our analysis and discuss these in the light of current burst oscillation models. Additionally, we investigate the bursts of two sources without previously detected oscillations. Despite that these sources have been observed at accretion rates where burst oscillations might be expected, we find their behaviour to be not anomalous compared to oscillation sources.

17 pages, 10 figures, accepted for publication in ApJ

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• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
• Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52
• Relativistic ocean $r$-modes during type-I X-ray bursts
• Thermonuclear X-ray bursts from LMXB 4U 1636$-$536 observed with AstroSat
• Waves in Thin Oceans on Oblate Neutron Stars
• Thermonuclear Type-I X-ray Bursts and Burst Oscillations from the Eclipsing AMXP Swift J1749.4-2807
• A search for the $835\text{Hz}$ superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536