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

TL;DR

This study investigates the orbital and spectral variability of the high mass X-ray pulsar GX 301--2 using Insight-HXMT data, revealing cyclotron lines, magnetic field estimates, and matter distribution around the neutron star.

Contribution

It provides new insights into the spectral features, magnetic field, and matter distribution of GX 301--2, including the detection of two cyclotron lines and their relationship with luminosity.

Findings

Detection of two cyclotron absorption lines at 30--42 keV and 48--56 keV.

Correlation between cyclotron line energies and X-ray luminosity.

Variation in column density near periastron correlates with flux.

Abstract

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 ($NH_{2}$), 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.