Author： Altamirano D. Watts A. Linares M. Markwardt C. B. Strohmayer T. Patruno A.
Publisher： Oxford University Press
ISSN： 0035-8711
Source： Monthly Notices of the Royal Astronomical Society, Vol.409, Iss.3, 2010-12, pp. : 1136-1145
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Abstract
ABSTRACTWe report the discovery of burst oscillations at the spin frequency in ten thermonuclear bursts from the accreting millisecond X-ray pulsar (AMXP) IGR J 17511− 3057. The burst oscillation properties are, like those from the persistent AMXPs SAX J 1808.4− 3658 and XTE J 1814− 338, anomalous compared to burst oscillations from intermittent pulsars or non-pulsing low-mass X-ray binaries. Like SAX J 1808.4− 3658 they show frequency drifts in the rising phase rather than the tail. There is also evidence for harmonic content. Where IGR J 17511− 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 J 17511− 3057 shows 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.
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