Pulsar Vela X-1 Pulse-to-Pulse Approach

Our results provide the first confirmation that the long-term decay of the harmonic line energy (E_cyc_,H) in Vela X-1 has ended.

We further report the first detection of a transient increase in E_cyc_,H between 2020 and 2023, which suggests a sudden and significant change in the magnetic field configuration or accretion geometry. In addition, E_cyc_,H shows slightly lower values at low luminosities and tends to flatten at higher luminosities, in the range of (0.13-1.21)x10^37^erg/s. The fundamental line energy (E_cyc_,F) exhibits no significant variation with time or luminosity, remaining stable at approximately 25keV.