Variations of cyclotron resonant scattering features in Vela X-1 revealed with Insight-HXMT

We present a detailed study of the high-mass X-ray binary Vela X-1, using observations performed by Insight-HXMT in 2019 and 2020, concentrating on timing analysis and spectral studies including pulse phase-resolved spectroscopy. The cyclotron line energy is found to be similar to 21-27 and 43-50 keV for the fundamental and first harmonic, respectively. We present the evolution of spectral parameters and find that the two line centroid energy ratio E-2/E-1 evolved from similar to 2 before MJD 58900 to similar to 1.7 after that. The harmonic cyclotron line energy has no relation to the luminosity but the fundamental line energy shows a positive correlation with X-ray luminosity, suggesting that Vela X-1 is located in the sub-critical accreting regime. In addition, the pulse phase-resolved spectroscopy in Vela X-1 is performed. Both the cyclotron resonant scattering feature and continuum parameters show strong variability over the pulse phase with the ratio of two line energies about 2 near the peak phases and down to similar to 1.6 around off-peak phases. Long-term significant variations of the absorption column density and its evolution over the pulse phase may imply the existence of the clumpy wind structure near the neutron star.

Automated summary

We conducted a detailed study of Vela X-1, a high-mass X-ray binary, using observations from Insight-HXMT in 2019 and 2020. Our analysis focused on timing analysis and spectral studies, including pulse phase-resolved spectroscopy. The cyclotron line energy was found to be 21-27 keV for the fundamental and 43-50 keV for the first harmonic. The study showed that the fundamental line energy correlates with X-ray luminosity, indicating that Vela X-1 is in the sub-critical accreting regime. The pulse phase-resolved spectroscopy revealed significant variability in both the cyclotron resonant scattering feature and continuum parameters, as well as long-term variations in the absorption column density over the pulse phase, possibly suggesting the presence of a clumpy wind structure near the neutron star.