Timing and spectral analysis of the 2025 outburst of 4U 1630−47 with NICER

We analyzed NICER observations of the 2025 outburst of the black hole X-ray binary 4U 1630−47 to investigate the spectral–timing properties of its transient low-frequency quasi-periodic oscillations (QPOs) and millihertz-scale quasi-regular modulation (QRM). During the rising phase of the outburst, the QPO centroid frequency increased from ∼0.24 Hz to ∼3.43 Hz. Wavelet-based state separation shows that the with-QPO intervals are associated with a higher inner disk temperature and a lower diskbb normalization than the without-QPO intervals, while the photon index (Γ) shows weaker changes within the uncertainties. Near the outburst peak, the source displayed a weak QRM at ∼0.07 Hz with a fractional rms amplitude of ∼4.7%, lower than that of the heartbeat state observed in 2023. Phase-resolved Hilbert–Huang analysis shows that the inner disk temperature is positively correlated with the X-ray flux, the diskbb normalization is anticorrelated, and Γ varies only weakly. Overall, the short-timescale spectral–timing variability is expressed most clearly through the disk-related parameters. The transient QPOs are therefore consistent with short-timescale disk-related variability during the rising phase, whereas the millihertz-scale QRM may represent a weaker heartbeat-like variability mode appearing near the outburst peak.