Abstract

We report a timing and spectral study of the 2024 outburst of the Be/X-ray binary pulsar 2S 1553-542 using NuSTAR and NICER observations. From the NuSTAR light curve, we measure a pulse period of 9.285022±0.000001 s. The energy-resolved pulse profiles are dominated by a single peak and show a wing-like structure most clearly in the 12–22 keV band. The pulsed fraction remains above 60% and increases with energy. The phase-averaged NuSTAR spectrum is described by an absorbed blackbody plus cutoff power-law continuum, together with an iron emission line and a cyclotron absorption feature. Using the cyclabs model, we obtain a cyclotron energy of Ecyc≃24.1 keV, corresponding to a magnetic field strength of B∼3×1012 G. Phase-resolved spectroscopy shows that the continuum and cyclotron-line parameters vary with pulse phase, and that the line becomes poorly constrained around the pulse-wing phase. We also searched the short NICER GTIs for transient mHz variability using wavelet analysis and a CEEMDAN-based Hilbert–Huang transform. Localized excesses near ∼10 mHz and ∼20 mHz are found, but the short exposures, COI effects, red-noise fluctuations, and the lack of a well-constrained Fourier peak limit their significance. We therefore treat them as candidate mHz variability rather than firm mHz QPO detections.

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