Accreting neutron stars in high-mass X-ray binaries (HMXBs) possess intense magnetic fields that fundamentally shape their electromagnetic output. Modulated by orbital motion and pulsar rotation, their light curves exhibit variability across timescales from milliseconds to years, with energy-dependent signatures that reflect the complex physics of the accretion column and surrounding circumstellar material.
In this presentation, I will show how to move beyond standard phase-averaged and phase-resolved spectroscopy by investigating pulsed fraction spectra—the energy-dependent fractional amplitude of the periodically modulated signal. I will present our recently updated analysis pipeline that integrates NuSTAR and XMM-Newton/pn data to construct truly broadband pulsed fraction spectra. I will demonstrate that our pipeline can robustly identify and interpret “dilution” signatures within these spectra. By quantifying how specific features—such as cyclotron resonant scattering features (CRSFs) and low-Z fluorescence lines—suppress or enhance the pulsed amplitude, we are now in a position to interpret combined timing-spectral information in a self-consistent physical picture. Finally, I will highlight how, thanks to the extraordinary energy resolution of XRISM/Resolve, it is possible to probe the fine structure of the modulated signal with unprecedented precision.