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Register a new userVariability study of the High Mass X-ray Binary IGR J18027--2016 with {it Swift}--XRT
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We report the results from pulsations and spectral analysis of a large number of observations of the HMXB pulsar IGR J18027--2016 with {it Swift}--XRT, carried out at different orbital phases. In some orbital phases, as seen in different XRT observations, the X-ray intensity is found to vary by a large factor, of about $sim$50. In all the observations with sufficient number of source X-ray photons, pulsations have been detected around the previously known pulse period of $sim$140 sec, When detected, the pulse profiles do not show any significant variation over a flux difference of a factor of $sim$3. The absorption column density is found to be large before and after the eclipse. We discuss various possible reasons for intensity and spectral variations in IGR J18027--2016, such as clumpy wind and hydrodynamic instabilities.
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The orbital profile of the High Mass X-ray binary IGR J16393-4643 shows a dip in its X-ray intensity, which was previously interpreted as an eclipse. Unlike most eclipsing HMXBs, where the X-ray eclipses are about two orders of magnitude fainter compared to the out of eclipse emission, this particular eclipse like feature is narrow and partial, casting doubt if it is indeed an eclipse. To further investigate the nature of this low intensity orbital phase, we use a large number of observations with Swift-XRT, covering the entire orbital phase. The soft X-ray observations also show this low intensity phase, which is about 30% of the intensity during rest of the orbit. We also carried out orbital phase resolved spectroscopy to compare the change in the spectral parameters inside and outside of this low intensity state. The results indicate that this low intensity state might not be an eclipse, as previously thought but absorption in the stellar corona. We have also provided the inclination angle of the binary for grazing eclipse caused by the stellar corona.
IGR J18027-2016 is an obscured high-mass X-ray binary formed by a neutron star accreting from the wind of a supergiant companion with a $sim$4.57 day orbital period. The source shows an asymmetric eclipse profile that remained stable across several years. We aim at investigating the geometrical and physical properties of stellar wind structures formed by the interaction between the compact object and the supergiant star. In this work we analyse the temporal and spectral evolution of this source along its orbit using six archival XMM-Newton observations and the accumulated Swift/BAT hard X-ray light curve. XMM-Newton light curves show that the source hardens during the ingress and egress of the eclipse, in accordance with the asymmetric profile seen in Swift/BAT data. A reduced pulse modulation is observed on the ingress to the eclipse. We model XMM-Newton spectra by means of a thermally-comptonized continuum (nthcomp) adding two gaussian emission lines corresponding to Fe K$alpha$ and Fe K$beta$. We included two absorption components to account for the interstellar and intrinsic media. We found that the local absorption column outside the eclipse fluctuates uniformly around $sim$ 6$times$10$^{22}$~cm$^{-2}$, whereas, when the source enters and leaves the eclipse, the column increases by a factor of $gtrsim$3, reaching values up to $sim$35 and $sim$15$times 10^{22}$~cm$^{-2}$, respectively. Combining the physical properties derived from the spectral analysis, we propose a scenario where a photo-ionisation wake (mainly) and an accretion wake (secondarily) are responsible for the orbital evolution of the absorption column, the continuum emission and the variability seen at the Fe-line complex.
IGR J16195-4945 is a hard X-ray source discovered by INTEGRAL during the Core Program observations performed in 2003. We analyzed the X-ray emission of this source exploiting the Swift-BAT survey data from December 2004 to March 2015, and all the available Swift-XRT pointed observations. The source is detected at a high significance level in the 123-month BAT survey data, with an average 15-150 keV flux of the source of ~1.6 mCrab. The timing analysis on the BAT data reveals with a significance higher than 6 standard deviations the presence of a modulated signal with a period of 3.945 d, that we interpret as the orbital period of the binary system. The folded light curve shows a flat profile with a narrow full eclipse lasting ~3.5% of the orbital period. We requested phase-constrained XRT observations to obtain a more detailed characterization of the eclipse in the soft X-ray range. Adopting resonable guess values for the mass and radius of the companion star, we derive a semi-major orbital axis of ~31 R_sun, equivalent to ~1.8 times the radius of the companion star. From these estimates and from the duration of the eclipse we derive an orbital inclination between 55 and 60 degrees. The broad band time-averaged XRT+BAT spectrum is well modeled with a strongly absorbed flat power law, with absorbing column N_H=7x 10^22 cm^(-2) and photon index Gamma=0.5, modified by a high energy exponential cutoff at E_cut=14 keV.
The source IGR J17200-3116 was discovered in the hard X-ray band by INTEGRAL. A periodic X-ray modulation at ~326 s was detected in its Swift light curves by our group (and subsequently confirmed by a Swift campaign). In this paper, we report on the analysis of all the Swift observations, which were collected between 2005 and 2011, and of a ~20 ks XMM-Newton pointing that was carried out in 2013 September. During the years covered by the Swift and XMM-Newton observations, the 1-10 keV fluxes range from ~1.5 to 4E-11 erg/cm^2/s. IGR J17200-3116 displays spectral variability as a function of the pulse phase and its light curves show at least one short (a few hundreds of seconds) dip, during which the flux dropped at 20-30% of the average level. Overall, the timing and spectral characteristics of IGR J17200-3116 point to an accreting neutron star in a high-mass system but, while the pulse-phase spectral variability can be accounted for by assuming a variable local absorbing column density, the origin of the dip is unclear. We discuss different possible explanations for this feature, favouring a transition to an ineffective accretion regime, instead of an enhanced absorption along the line of sight.
Supergiant X-ray binaries usually comprise a neutron star accreting from the wind of a OB supergiant companion. They are classified as classical systems and the supergiant fast X-ray transients (SFXTs). The different behavior of these sub-classes of sources in X-rays, with SFXTs displaying much more pronounced variability, is usually (at least) partly ascribed to different physical properties of the massive star clumpy stellar wind. In case of SFXTs, a systematic investigation of the effects of clumps on flares/outbursts of these sources has been reported by Bozzo et al. (2017) exploiting the capabilities of the instruments on-board XMM-Newton to perform a hardness-resolved spectral analysis on timescales as short as a few hundreds of seconds. In this paper, we use six XMM-Newton observations of IGR J18027-2016 to extend the above study to a classical supergiant X-ray binary and compare the findings with those derived in the case of SFXTs. As these observations of IGR J18027-2016 span different orbital phases, we also study its X-ray spectral variability on longer timescales and compare our results with previous publications. Although obtaining measurements of the clump physical properties from X-ray observations of accreting supergiant X-ray binaries was already proven to be challenging, our study shows that similar imprints of clumps are found in the X-ray observations of the supergiant fast X-ray transients and at least one classical system, i.e. IGR J18027-2016. This provides interesting perspectives to further extend this study to many XMM-Newton observations already performed in the direction of other classical supergiant X-ray binaries.
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