Cyclotron Line Variability and Accretion Dynamics in Vela X-1
We present a comprehensive analysis of Vela X-1 using two new NuSTAR observations, placed in the context of four earlier datasets obtained between 2012 and 2020. The energy-resolved pulse profiles demonstrate a significant transformation from an asymmetric structure at low energies to distinct double peaks above 12 keV, whereas the pulse fraction escalates with photon energy but decreases with flux. Broadband spectra validate the Fe K alpha emission line and disclose both fundamental and harmonic cyclotron resonant scattering characteristics (CRSF). We observe no substantial link between CRSF energies and luminosity, contrary to previous findings; rather, the photon index and folding energy demonstrate distinct anti-correlations with flux, aligning with sub-critical accretion and increased Comptonization in the accretion column. Our results provide the first clear evidence that the harmonic CRSF in Vela X-1 does not follow the long-term decay previously claimed. The fundamental line energy also displays an irregular evolution, without a clear monotonic trend. Notably, the harmonic-to-fundamental energy ratio departs from the canonical value of two, suggesting that the line-forming regions are located at different heights within the accretion column. These results provide new constraints on the accretion geometry and magnetic field topology of Vela X-1, highlighting the importance of continued monitoring with current and future X-ray observatories.