2025ApJ...995..153V

We present a detailed temporal study of a type I X-ray burst from the neutron star low-mass X-ray binary (NS-LMXB) XTE J1810-189, observed on 2023 April 27 using the Neutron Star Interior Composition Explorer. The burst exhibited a rapid rise time of 2.55 s, followed by an exponential decay lasting for 7.5 s, with a total duration of ∼13 s. Type I X-ray bursts are driven by thermonuclear burning on the surface of a neutron star in an NS-LMXB. As these bursts originate from the stellar surface they can exhibit highly coherent signals known as burst oscillations, which serve as probes of the neutron star's spin frequency. We report the detection of a burst oscillation signal at ∼459 Hz at the cooling tail of the burst. The oscillation showed a strong Leahy-normalized power of P_L = 35.95 at 458.92 Hz, corresponding to a single-trial significance of 5.53σ and a multiple-trial corrected significance of 3.14σ. The folded pulse profile in the 0.2-12 keV band is well described by a constant plus sinusoid with a fractional rms amplitude of 14.63%. These results suggest that the burst oscillation frequency of XTE J1810-189 directly reflects on the neutron star's spin, measured here to be ∼2.18 ms, placing it among the rapidly rotating NS-LMXBs. This burst oscillation signal at the cooling tail of the burst can be interpreted through surface mode model or the asymmetric cooling wake model.