The elusive cyclotron line in 4U 1901+03: Hidden, yet present

Context. Cyclotron resonant scattering features in accreting X-ray pulsars are often difficult to detect, especially when shallow or variable. Recent studies have shown that combining spectral and timing analyses enhances their detectability. Aims. We investigated the evolution of energy-resolved pulse profiles of the X-ray pulsar 4U 1901+03 during its 2019 giant outburst, focusing on the 30-40 keV range where there have been disputed claims of a cyclotron line detection and its properties. Methods. We analysed four NuSTAR observations of 4U 1901+03 at different luminosities. We studied energy-resolved pulse profiles using harmonic decomposition, cross-correlation analysis, energy-phase maps, and pulsed-fraction spectra, and we used Bayesian spectral modelling to assess the presence and properties of a cyclotron line. Results. We detected significant spectral-timing variability in the 30-40 keV range, which became stronger at lower luminosities. We found a pronounced drop in the pulsed fraction near approximate to 35 keV only in the lowest accretion state and in the first harmonic of one intermediate-luminosity observation. Adopting a Bayesian informative approach on the spectral parameters, we find evidence of a cyclotron line in all the examined energy spectra, with an average centroid energy of E-cyc approximate to 32 keV, varying by only approximate to 1.6%, with anti-correlation between line depth and luminosity. We show that a combined spectral-timing approach is more sensitive than phase-averaged spectroscopy to shallow cyclotron features. The luminosity-dependent evolution of pulse profiles and the depth of the cyclotron line point to a drastic change in the emission geometry and the accretion flow structure.