NICER Observation of Unusual Burst Oscillations in 4U 1728-34
arXiv:1908.01206 · doi:10.3847/1538-4357/ab20c4
NICER observed seven thermonuclear X‑ray bursts from the neutron star 4U 1728‑34, finding three burst oscillations in the burst tails, two of which have unusually large amplitudes and appear only at photon energies above 6 keV.
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.
14 pages, 6 figures, published in ApJ 878, 2, 145