Probing the X-ray Spectral and Timing Behaviour of the High-Mass X-ray Binaries EXO 2030+375 and Vela X-1

This work presents spectral and timing studies of two accreting X-ray pulsars, the Be/X-ray binary EXO 2030+375 and the wind-fed system Vela X-1, based on multi-mission X-ray observations. For EXO 2030+375, luminosity- and energy-dependent pulse-profile evolution and spectral behavior reveal transitions between different accretion regimes during outbursts. For Vela X-1, long-term monitoring and pulse-to-pulse analyses show complex evolution of cyclotron line energies, including a recent transient increase of the harmonic line, indicating changes in the magnetic-field configuration.