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

## TL;DR

NICER observed seven thermonuclear X-ray bursts from 4U 1728-34, discovering unusual high-energy, high-amplitude burst oscillations that challenge existing theoretical models of neutron star burst behavior.

## Contribution

This study reports the detection of atypical burst oscillations with large amplitudes and energy dependence, providing new insights into neutron star burst phenomena.

## Key findings

- Two bursts show large fractional rms amplitudes (~48%) at energies above 6 keV.
- One burst's oscillation matches previous observations with lower amplitude (~7.7%).
- Large-amplitude oscillations are difficult to explain with current models.

## 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.