The study of X-ray reprocessing is one of the key diagnostic tools to probe the environment in X-ray binary systems. One difficult aspect of studying X-ray reprocessing is the presence of much brighter primary radiation from the compact star together with the reprocessed radiation. In contrast for eclipsing systems, the X-rays we receive during eclipse are only those produced by reprocessing of the emission from the compact star by the surrounding medium. We report results from a spectral study of the X-ray emission during eclipse and outside eclipse (when available) in 9 high mass X-ray binaries (HMXBs) with XMM-Newton EPIC pn to investigate different aspects of the stellar wind in these HMXBs. During eclipse the continuum component of the spectrum is reduced by a factor of ∼8-237, but the count-rate for 6.4 keV Iron emission line or complex of Iron emission lines in HMXBs are reduced by a smaller factor leading to large equivalent widths of the Iron emission lines. This indicates a large size for the line emission region, comparable to or larger than the companion star in these HMXB systems. However there are significant system to system differences. 4U 1538−52, in spite of having a large absorption column density, shows a soft emission component with comparable flux during the eclipse and out-of-eclipse phases. Emission from Hydrogen-like Iron has been observed in LMC X-4 for the first time, in the out-of-eclipse phase in one of the observations. Overall, we find significant differences in the eclipse spectrum of different HMXBs and also in their eclipse spectra against out-of-eclipse spectra.

We report the results from pulsations and spectral analysis of a large number of observations of the HMXB pulsar IGR J18027-2016 with 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 ∼50. In all the observations with sufficient number of source X-ray photons, pulsations have been detected around the previously known pulse period of ∼140 sec, When detected, the pulse profiles do not show any significant variation over a flux difference of a factor of ∼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.

We present a comprehensive temporal and spectral analysis of the ‘softer’ variability classes (i.e. θ, β, δ, ρ, κ, ω and γ) of the source GRS 1915+105 observed by AstroSat during the 2016−2021 campaign. Wide-band (3−60 keV) timing studies reveal the detection of high-frequency quasi-periodic oscillations (HFQPOs) with frequencies of 68.14−72.32 Hz, significance of 2.75−11σ and rms amplitude of 1.48–2.66 per cent in δ, κ, ω and γ variability classes. Energy-dependent power spectra show that HFQPOs are detected only in the 6−25 keV energy band and rms amplitude is found to increase (1–8 per cent) with energy. The dynamical power spectra of the κ and ω classes demonstrate that HFQPOs seem to be correlated with high count rates. We observe that wide-band (0.7−50 keV) energy spectra can be described by the thermal Comptonization component (nthComp) with a photon index (Γnth) of 1.83−2.89 along with an additional steep (ΓPL ∼ 3) power-law component. The electron temperature (kTe) of 1.82−3.66 keV and optical depth (τ) of 2−14 indicate the presence of a cool and optically thick corona. In addition, nthComp components, 1.97 ≲ Γnth ≲ 2.44 and 1.06 × 10−8 ≲ Fnth (erg cm−2 s−1) ≲ 4.46 × 10−8, are found to dominate in the presence of HFQPOs. Overall, these findings infer that HFQPOs are possibly the result of the modulation of the ‘Comptonizing corona’. Further, we find that the bolometric luminosity (0.3−100 keV) of the source lies within the sub-Eddington (3–34 per cent LEdd) regime. Finally, we discuss and compare the obtained results in the context of existing models on HFQPOs.

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