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arXiv · 1908.01206

NICER Observation of Unusual Burst Oscillations in 4U 1728-34

Simin Mahmoodifar·Tod E. Strohmayer·Peter Bult·Diego Altamirano·Zaven Arzoumanian·Deepto Chakrabarty·Keith C. Gendreau·Sebastien Guillot·Jeroen Homan·Gaurava K. Jaisawal·Laurens Keek·Michael T. Wolff
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Abstract

The Neutron Star Interior Composition Explorer (NICER) has observed seven thermonuclear X-ray bursts from the Low Mass X-ray Binary (LMXB) neutron star 4U 1728-34 from the start of the mission's operations until February of 2019. Three of these bursts show oscillations in their decaying tail with frequencies that are within 1 Hz of the previously detected burst oscillations from this source. Two of these burst oscillations have unusual properties: They have large fractional rms amplitudes of $ 48 \pm 9 \%$ and $ 46 \pm 9 \%$, and they are detected only at photon energies above 6 keV. By contrast, the third detected burst oscillation is compatible with previous observations of this source, with a fractional rms amplitude of $7.7 \pm 1.5\%$ rms in the 0.3 to 6.2 keV energy band. We discuss the implications of these large-amplitude burst oscillations, finding they are difficult to explain with the current theoretical models for X-ray burst tail oscillations.

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Simin Mahmoodifar, Tod E. Strohmayer, Peter Bult, Diego Altamirano, Zaven Arzoumanian, Deepto Chakrabarty, Keith C. Gendreau, Sebastien Guillot, Jeroen Homan, Gaurava K. Jaisawal, Laurens Keek, Michael T. Wolff. 2019-08-03. NICER Observation of Unusual Burst Oscillations in 4U 1728-34. https://doi.org/10.3847/1538-4357/ab20c4

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