Type I X-ray bursts and burst oscillations in the accreting millisecond X-ray pulsar IGR J17511-3057

TL;DR

This paper reports the discovery of burst oscillations at the spin frequency in ten thermonuclear bursts from the accreting millisecond X-ray pulsar IGR J17511-3057, revealing unique properties and implications for burst oscillation models.

Contribution

It presents the first detection of burst oscillations in IGR J17511-3057 and compares their properties with other known accreting millisecond X-ray pulsars, highlighting novel behaviors.

Findings

Burst oscillations detected in 10 bursts from IGR J17511-3057.

Oscillation properties are similar to some known pulsars but show unique patterns.

No evidence of photospheric radius expansion, setting an upper distance limit.

Abstract

We report the discovery of burst oscillations at the spin frequency in ten thermonuclear bursts from the accreting millisecond X-ray pulsar (AMXP) IGR J17511-3057. The burst oscillation properties are, like those from the persistent AMXPs SAX J1808.4-3658 and XTE J1814-338, anomalous compared to burst oscillations from intermittent pulsars or non-pulsing LMXBs. Like SAX J1808.4-3658 they show frequency drifts in the rising phase rather than the tail. There is also evidence for harmonic content. Where IGR J17511-3057 is unusual compared to the other two persistent pulsars is that oscillations are not detected throughout all bursts. As accretion rate drops the bursts get brighter and their rise/decay time scales become shorter, while the oscillation amplitude falls below the detection threshold: first in the burst peak and then also in the rise. None of the bursts from IGR J17511-3057 show evidence for photospheric radius expansion (which might be expected to suppress oscillation amplitude) which allow us to set an upper limit to the distance of 6.9 kpc. We discuss the implications of our results for models of the burst oscillation mechanism.