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Register a new userTiming and spectral variability of high mass X-ray pulsar GX 301--2 over orbital phases observed by Insight-HXMT
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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 lines 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.
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We investigate the absorption and emission features in observations of GX 301-2 detected with Insight-HXMT/LE in 2017-2019. At different orbital phases, we found prominent Fe Kalpha, Kbeta and Ni Kalpha lines, as well as Compton shoulders and Fe K-shell absorption edges. These features are due to the X-ray reprocessing caused by the interaction between the radiation from the source and surrounding accretion material. According to the ratio of iron lines Kalpha and Kbeta, we infer the accretion material is in a low ionisation state. We find an orbital-dependent local absorption column density, which has a large value and strong variability around the periastron. We explain its variability as a result of inhomogeneities of the accretion environment and/or instabilities of accretion processes. In addition, the variable local column density is correlated with the equivalent width of the iron Kalpha lines throughout the orbit, which suggests that the accretion material near the neutron star is spherically distributed.
GX 301-2 provides a rare opportunity to study both disk and wind accretion in a same target. We report Insight-HXMT observations of the spin-up event of GX 301-2 happened in 2019 and compare with those of wind-fed state. The pulse profiles of the initial rapid spin-up period are dominated by one main peak, while those of the later slow spin-up period are composed of two similar peaks, as those of wind-fed state. These behaviors are confirmed by Fermi/GBM data, which also show that during the rapid spin-up period, the main peak increases with luminosity up to $8times10^{37}$ erg s$^{-1}$, but the faint peak keeps almost constant. The absorption column densities during the spin-up period are $sim1.5times10^{23}$ cm$^{-2}$, much less than those of wind-fed state at similar luminosity ($sim9times10^{23}$ cm$^{-2}$), supporting the scenario that most of material is condensed into a disk during the spin-up period. We discuss possible differences between disk and wind accretion that may explain the observed different trend of pulse profiles.
Recently a retrograde neutron star is proposed for the classical wind-fed X-ray pulsar, GX 301-2, to explain the orbital spin-up to spin-down reversal near periastron, based on the stream model invoked to explain the pre-periastron flare of GX 301-2 previously. We study in detail three rare spin-up events detected by Fermi/GBM and find that the spin derivatives are correlated with the Swift/BAT fluxes, following a relation of $dot{ u}propto F^{0.75pm0.05}$. All the spin-up events of GX 301-2 started about 10 days after the periastron, which is the time needed for tidally stripped gas to reach the neutron star. The slow rotation of the optical companion implies that the accreted matter is likely to have angular momentum in the direction of the orbital motion, as in a Roche-Lobe-like overflow. As a result, the spin-up events of GX 301-2 would favor accretion of a prograde disk to a prograde neutron star. We also find that the flare of intrinsic X-ray emission of GX 301-2 happened 0.4 days before periastron, while the flare of low energy emission (2-10 keV) happened about 1.4 days before periastron. The preceding low energy flare can be explained by stronger absorption of the intrinsic X-ray emission closer to the periastron. This finding weakened the need of the stream model. The pulse fraction of GX 301-2 near periastron is reduced heavily, which is likely caused by Compton scattering process. Compton reflection from the optical companion might be responsible for the observed orbital spin reversal of GX 301-2.
In this paper, we presented a detailed timing analysis of a prominent outburst of 4U 0115+63 detected by textit{Insight}-HXMT in 2017 August. The spin period of the neutron star was determined to be $3.61398pm 0.00002$ s at MJD 57978. We measured the period variability and extract the orbital elements of the binary system. The angle of periastron evolved with a rate of $0.048pm0.003$ $yr^{-1}$. The light curves are folded to sketch the pulse profiles in different energy ranges. A multi-peak structure in 1-10 keV is clearly illustrated. We introduced wavelet analysis into our data analysis procedures to study QPO signals and perform a detailed wavelet analysis in many different energy ranges. Through the wavelet spectra, we report the discovery of a QPO at the frequency $sim 10$ mHz. In addition, the X-ray light curves showed multiple QPOs in the period of $sim 16-32 $ s and $sim 67- 200 $ s. We found that the $sim100$ s QPO was significant in most of the observations and energies. There exist positive relations between X-ray luminosity and their Q-factors and S-factors, while the QPO periods have no correlation with X-ray luminosity. In wavelet phase maps, we found that the pulse phase of $sim 67- 200 $ s QPO drifting frequently while the $sim 16-32 $ s QPO scarcely drifting. The dissipation of oscillations from high energy to low energy was also observed. These features of QPOs in 4U 0115+63 provide new challenge to our understanding of their physical origins.
We aim at characterising the structure of the gas and dust around the high mass X-ray binary GX 301-2, a highly obscured X-ray binary hosting a hypergiant star and a neutron star, in order to better constrain its evolution. We used Herschel PACS to observe GX 301-2 in the far infrared and completed the spectral energy distribution of the source using published data or catalogs, from the optical to the radio range (0.4 to 4x10^4 micrometer). GX 301-2 is detected for the first time at 70 and 100 micrometer. We fitted different models of circumstellar environments to the data. All tested models are statistically acceptable, and consistent with a hypergiant star at ~3 kpc. We found that the addition of a free-free emission component from the strong stellar wind is required and could dominate the far infrared flux. Through comparisons with similar systems and discussion on the estimated model parameters, we favour a disk-like circumstellar environment of ~8 AU that would enshroud the binary system. The temperature goes down to ~200 K at the edge of the disk, allowing for dust formation. This disk is probably a rimmed viscous disk with an inner rim at the temperature of the dust sublimation temperature (~1500 K). The similarities between the hypergiant GX 301-2, B[e] supergiants and the highly obscured X-ray binaries (in particular IGR J16318-4848) are strengthened. GX 301-2 might represent a transition stage in the evolution of massive stars in binary systems, connecting supergiant B[e] systems to luminous blue variables.
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