Type-I X-ray burst oscillations are powered by thermonuclear energy released on the neutron star (NS) surface in low-mass X-ray binaries (LMXBs), where the burst oscillation frequencies are close to the NS spin rates. In this work, we report the detection of oscillation at 584.65 Hz during the cooling tail of type-I X-ray bursts observed from the accreting NS LMXB 4U 1730–22 on 2022 March 20, by the Neutron star Interior Composition Explorer telescope. The oscillation signal showed a strong Leahy power, P m ∼ 54.04, around 584.65 Hz, which has single-trial and multiple-trial confidence levels of 7.05σ and 4.73σ, respectively. The folded pulse profile of the oscillation in the 0.2–10 keV band showed a sinusoidal shape with the fractional rms amplitude of (8.0 ± 1.1)%. We found the oscillation frequency showed insignificant upward drifting, i.e., less than 0.3 Hz, during the cooling tail, similar to the behavior appearing in accreting millisecond X-ray pulsars (AMXP), and indicate the source could be an AMXP spinning at 1.71 ms.

NICER observed two outbursts from the neutron star low-mass X-ray binary 4U 1730–22 in 2021 and 2022, which showed a similar spectral evolution in the hardness-intensity diagram. Seventeen type I X-ray bursts were identified in both outbursts. The X-ray burst spectra showed clear deviations from the blackbody model, firstly ∼10 s after onset. Adding the enhanced persistent emission due to the Poynting-Robertson drag or the reflection from the accretion disk both significantly improved the fitting results. We found that 12 out of 17 X-ray bursts showed the photospheric radius expansion (PRE) characteristic. Considering the nine PRE bursts out of ten X-ray bursts observed by Insight-HXMT, 78% of bursts from 4U 1730–22 exhibited PRE. According to the burst rise time, the duration, the local accretion rate, and the burst fuel composition estimated from recurrence time, we propose that these PRE bursts were powered by pure helium. From the touchdown flux of PRE bursts, we estimate the source distance of d = 7.54 ± 0.46(X = 0) kpc for a canonical neutron star with MNS = 1.4 M⊙ and RNS = 10 km.