NICER / ISS Science Nugget for July 5, 2018

NICER Observes Unusual Burst Oscillations in 4U 1728-34

Type I X-ray bursts are thermonuclear explosions on the surfaces of neutron stars found in Low Mass X-ray Binaries (LMXBs). Lasting tens of seconds, these unpredictable flashes in X-rays are due to unstable burning of material accreted from a companion star.

Coherent pulsations in brightness, known as burst oscillations, have been observed in the rise and/or tail of some X-ray bursts. In 2003, the discovery of burst oscillations at a frequency of 401 Hz from the accretion-powered pulsar SAX J1808.4-3658 revealed that the oscillation frequency was very close to the star's known spin frequency, establishing burst oscillations as nuclear-powered pulsations that can be used to estimate the spin rates of other accreting neutron stars.

The fractional root-mean-squared (rms) amplitude of burst oscillation signals is usually between 5% and 20%. There are some cases where larger amplitudes have been observed in the rising phase of a burst, but burst oscillations seen in the tail usually have smaller amplitudes. NICER has observed an unusually large-amplitude burst oscillation, one that occurred in the tail of a burst from the LMXB 4U 1728-34, with a fractional rms of 51.7 ± 9.5%. This is the largest amplitude ever observed for oscillations in a burst tail. The fact that this signal has been observed only at high X-ray energies (above 6 keV) and with such a large amplitude is very puzzling, challenging our current theoretical models of thermonuclear explosions and flame propagation in the extreme gravitational, nuclear, and electromagnetic environment of a neutron star's surface.

Figure: (Upper panel) The solid trace shows the characteristic sudden onset and decaying tail of a Type I X-ray burst brightness profile or "lightcurve", referring to the right-hand vertical axis. The tight bundle of contours between the dashed vertical lines shows the result of a search for periodic fluctuations in brightness in narrow time intervals conducted across the entire burst; the periodic emission at 362.5 Hz (left vertical axis) was only seen in this part of the burst tail.
(Lower-left panel) A "periodogram" showing excess power at the pulsation frequency.
(Lower-right panel) The shape of the brightness oscillation at 362.5 Hz, with the best-fit sine curve in green.

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