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

We report the orbital X-ray variability of the high-mass X-ray binary (HMXB) GX 301-2. GX 301-2 underwent a spin-up process in 2018-2020 with the period evolving from similar to 685-670 s. The energy-resolved pulse profiles of the pulsar at 1-60 keV varied from single-peaked and sinusoidal shapes to multipeaked ones across different orbital phases. Pulse fractions evolving over the orbit had negative correlations with the X-ray flux. The broad-band X-ray energy spectrum of the pulsar can be described with a partially covered negative-positive cut-off power-law continuum model. Near the periastron passage of the pulsar we found strong variation in the additional column density (), which correlated with variation of the flux. Curves of growth for both FeK alpha and FeK beta lines were plotted to investigate the distribution of matter around the neutron star. We also found evidence for two cyclotron absorption lines in the phase-averaged spectra in GX 301-2, with one line of 30-42 keV and the other line varying over 48-56 keV. The centroid energies of both lines show a similar relationship with X-ray luminosity: positive correlation in the lower luminosity range, and a negative relation above a critical luminosity of . We estimate the surface magnetic field of the neutron star in GX 301-2 to be similar to(0.5-2) x 10(13) G. The two cyclotron line energies have a nearly fixed ratio of similar to 1.63 while having a low strength ratio (similar to 0.05), suggesting that these two features may actually be one line.

Automated summary

We report the orbital X-ray variability of the high-mass X-ray binary GX 301-2, which underwent a spin-up process from 2018 to 2020. The pulse profiles and fractions of the pulsar varied across different orbital phases, showing negative correlations with X-ray flux.