Timing and spectral variability of high mass X-ray pulsar GX 301--2 over orbital phases observed by Insight-HXMT

We report the orbital X-ray variability of high mass X-ray binary (HMXB) GX301–2. GX301–2 undergone a spin up process in 2018–2020 with the period evolving from $\sim$ 685 s to 670 s. The energy resolved pulse-profiles of the pulsar in 1–60 keV varied from single peaked and sinusoidal shapes to multi-peaked across different orbital phases. Pulse fractions evolving over orbit had negative correlations with the X-ray flux. The broad-band X-ray energy spectrum of the pulsar can be described with a partial covering negative positive cutoff power-law continuum model. Near the periastron passage of the pulsar we found a strong variation in the additional column density ($N{H}_2$), which correlated with variation of the flux. Curves of growth for both Fe K$\alpha$ and Fe K$\beta$ lines were plotted to investigate the distribution of matter around neutron star. We have also found the evidence of two cyclotron absorption lines in the phase-averaged spectra in GX301–2, with one line of 30–42 keV and the other line varying in 48–56 keV. Both two line’s centroid energies show the similar relationship with X-ray luminosity: positive correlation in lower luminosity range, and negative relation above a critical luminosity of ${10}^{37}$ erg s${}^{-1}$. We estimated the surface magnetic field of the neutron star in GX301–2 at ~$(0.5-2)\times {10}^{13}$ G. Two cyclotron line energies have a nearly fixed ratio of ~1.63 while having a low strength ratio (~0.05), suggesting that these two features may actually be one line.