Executive Summary : | Neutron stars (NSs) are astrophysical objects which display diverse emission behavior & extreme conditions of temperature, density and magnetic fields. They can occur as isolated objects or in gravitationally bound systems (X-ray binaries (XRBs)). As a result, NSs evolve individually as well as in conjunction with its partner. Subsequently, a study of NS environment and accretion flows around them is important ingredient for probing strong gravity regions & for finding answers to fundamental physics of dense matter which is not possible in terrestrial laboratory. Emission properties in XRBs depend on nature of compact object, geometry/ strength of magnetic field & rate of accretion flow. Observationally, these manifest in the form of intensity, spectral and time variability. The proposed work has multiple dimensions in order to achieve a more complete picture of accretion geometry & emission mechanism in X-ray pulsars. Foremost, pulse profiles are fingerprints of X-ray pulsars. They show long term variations with time and energy. Spin evolution is interpreted in terms of accretion torque & accretion disk-magnetosphere interaction. Energy-dependence of pulse profile is a direct reflection of changes in accretion geometry and it will shed light on viability of various possible physical processes such as cyclotron radiation, Bremsstrahlung and Comptonization occurring inside the accretion column. Another fundamental parameter that characterizes an XRB is its orbital period whose accurate measurement and evolution mechanism is of utmost concern for constraining the properties of XRBs. The orbital separation is expected to change due to several factors, but over the years, it has been found that the rate of orbital evolution of XRBs is much higher than that predicted by conventional mechanisms. Recent works have shown that orbital evolution in XRBs can show orbital glitches attributable to stellar magnetic convection, scattering events with objects around the system, a hierarchical triple body system and non-conservative mass transfer scenario. This work proposes to investigate these aspects in further detail. Stellar winds are often inhomogeneous in terms of wind density and presence of clumps of varying size. When NS happens to pass through this clumpy medium, it leads to reprocessing of the source photons to produce iron fluorescence line. If a significant fraction of source photons is scattered, then it can lead to formation of Compton shoulder. For high column densities, it is important to determine the phase dependence and relative ratio of different iron ionized species. CRSF is like show stealer in X-ray astronomy by being the only direct means for estimating the magnetic field strength of NSs. This acts as an indicator of accretion geometry and emission beam pattern. The main idea behind CRSF measurements is to study their pulse phase and luminosity dependence, trend in their line energy, shape and harmonic separations with energy and time. |