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

¹ INAF – IASF-Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy
² Department of Astronomy, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
³ Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁴ Dipartimento di Fisica e Chimica Emilio Segrè, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy
⁵ Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15784 Athens, Greece
⁶ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone, (RM), Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 October 2025
Accepted: 15 March 2026

Abstract

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 ≈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 ≈ 32 keV, varying by only ≈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.