Discovery of a 584.65 Hz Burst Oscillation in the Low Mass X-Ray Binary 4U 1730--22

TL;DR

This paper reports the detection of a 584.65 Hz burst oscillation from the neutron star LMXB 4U 1730--22, providing insights into neutron star spin behavior and accretion processes.

Contribution

First detection of a burst oscillation at 584.65 Hz in 4U 1730--22, suggesting the source may be an accreting millisecond X-ray pulsar with a spin period of 1.71 ms.

Findings

Oscillation frequency shows less than 0.3 Hz drift during cooling tail.

Oscillation has a sinusoidal profile with 12.5% rms amplitude.

Detection confidence levels are 7.05σ (single trial) and 4.78σ (multiple trials).

Abstract

Type-I X-ray burst oscillations are powered by thermonuclear 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 a type-I X-ray bursts observed from the accreting NS LMXB 4U~1730--22 in 2022 March 20, by the \textit{ Neutron star Interior Composition Explorer} (\textit{NICER}) telescope. The oscillation signal showed a strong Leahy power, $P_{\rm m}\sim54.04$, around 584.65 Hz, which has single trial and multiple trials confidence levels of $7.05\sigma$ and $4.78\sigma$, respectively. The folded pulse profile of the oscillation in the 0.2--10 keV band showed a sinusoidal shape with the fraction amplitude rms of $(12.5\pm1.8)\%$. We found the oscillation frequency showed insignificant upward drifting, i.e., less than 0.3 Hz, during the cooling tail, similar as the behavior appearing in accreting millisecond X-ray pulsars (AMXP), and indicate the source could be an AMXP spinning at 1.71 ms.