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

TL;DR

This study analyzes the spectral and timing properties of Vela X-1 using Insight-HXMT data, revealing variations in cyclotron lines, their relation to luminosity, and pulse phase-dependent spectral changes, indicating complex accretion and wind structures.

Contribution

It provides detailed pulse phase-resolved spectroscopy and long-term analysis of cyclotron lines, showing their evolution and correlation with luminosity in Vela X-1.

Findings

Cyclotron line energies are at ~21-27 keV and 43-50 keV for fundamental and harmonic.

The ratio of line energies evolved from ~2 to ~1.7 over time.

Fundamental line energy correlates positively with X-ray luminosity.

Abstract

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 at ~21-27 keV and 43-50 keV for the fundamental and first harmonic, respectively. We present the evolution of spectral parameters and find that two line centroid energy ratio E2/E1 evolved from ~2 before MJD 58900 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 CRSF 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 ~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.