Detection of possible burst oscillation in the neutron star low-mass X-ray binary 4U 1323-62

Abstract

Burst oscillations observed during thermonuclear X-ray bursts arise from asymmetric brightness patterns on the neutron star surface and provide a direct probe of the neutron star spin frequency. We present a detailed timing analysis of the neutron star low-mass X-ray binary 4U 1323-62 using 2024 observations with XMM-Newton and NuSTAR observatories. We identify nine thermonuclear X-ray bursts in the XMM-Newton/EPIC-pn data, along with eclipsing dips in the light curve. One of the XMM-Newton bursts exhibits a rare doublet structure. In addition, NuSTAR detects six bursts, four of which occur simultaneously with those observed with XMM-Newton. We identify a possible burst oscillation signal at $\sim$611.5 Hz in the XMM-Newton data. The strongest oscillation, detected during the primary burst of the doublet burst, reaches a maximum $Z_1^{2}$ power of $\sim35$. An analytical estimate accounting for the searched frequency and time intervals gives a significance of $\sim3.0σ$, whereas independent Monte Carlo simulations yield a more robust global significance of only $\sim2.4σ$. We therefore interpret the signal as a tentative detection of burst oscillation. The folded pulse profile in the 0.5-10 keV band is well described by a sinusoid, with a fractional rms amplitude of $\sim30\pm6$%. The oscillation frequency corresponds to a neutron-star spin period of $\sim$1.635 ms, suggesting that 4U 1323-62 may harbor a rapidly rotating millisecond neutron star.