Timing and Spectral Analysis of the 2024 Outburst of 2S 1553 - 542 with NuSTAR and NICER

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 plus or minus0.000001 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 E cyc 24.1 keV, corresponding to a magnetic field strength of B about3 times10 12 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 about10 mHz and about20 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.