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

TL;DR

This study demonstrates that combining spectral and timing analyses enhances the detection of elusive cyclotron lines in X-ray pulsars, revealing a consistent 32 keV feature in 4U 1901+03 during its 2019 outburst.

Contribution

We introduce a combined spectral-timing method that improves the detection sensitivity of cyclotron lines in accreting X-ray pulsars, especially at low luminosities.

Findings

Detected a cyclotron line at approximately 32 keV in all spectra.

Found an anti-correlation between line depth and luminosity.

Observed luminosity-dependent changes in pulse profiles and emission geometry.

Abstract

Context. Cyclotron resonant scattering features (CRSFs) 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. 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. We also 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 becomes 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, we find evidence for a cyclotron line in all examined energy spectra, with an average centroid energy of E_cyc approx 32 keV (varying by only 1.6%), and an anti-correlation between line depth and luminosity. Conclusions. 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 cyclotron line depth point to a drastic change in the emission geometry and accretion flow structure.