Research Projects

Low mass X-ray binaries (LMXBs) host a compact object (neutron star/black hole) and a donor star (main sequence/post main sequence star) in a gravitationally bound orbit. Matter accreted from the donor onto the compact object forms an accretion disk, which predominantly emits in X-rays. X-rays are also emitted due to Comptonization of seed photons by highly energetic electrons from the ‘corona’ around the compact object, the exact nature and properties of which are yet obscure. Relative contribution of X-rays from accretion disk and corona lead to different source states characterized by correlated spectral and temporal variabilities. Based on their correlated spectral and temporal properties, LMXBs are classified into various subclasses. The exact physical mechanisms that cause the observed variabilities in these subclasses are not well understood. Among the spectral models proposed to explain the observed spectral variations in these systems, there is no consensus regarding the most accurate model. Temporal properties include broadband noise and narrow quasi-periodic oscillations (QPOs) at characteristic frequencies, which act as probes into the innermost regions of accretion flow. In addition, QPOs exhibit positive or negative time lags between soft and hard photons, and studies on these can provide insights into the geometry of the system. Despite several studies on temporal features, their origin is still not clear. Moreover, the correlation of spectral and temporal properties with each other and with the system’s geometry, remains unclear, as there are very few models that explain spectral and temporal properties simultaneously. Currently several national and international research groups are involved in the development of more comprehensive spectral models in order to resolve the aforementioned issues. However, the robustness of these models needs to be tested using data from instruments with higher spectral and temporal capabilities. Most erstwhile studies have been limited by the spectro-temporal capabilities of previous missions. In addition, these studies have not leveraged the use of data from multiple missions, spanning over different epochs to investigate the properties of the sources in the broadband. Moreover, the use of single epoch data has also limited our understanding of these sources to only one source state at a time. The advent of new age X-ray observatories such as AstroSat, NICER, NuStar, Swift and XMM Newton has ushered in data with high spectral and temporal resolution. There exist vast repositories of data that are publicly available in the data archives of these missions, which remain unexplored. This gives much scope to revisit previously studied LMXBs and to explore newly discovered LMXBs with higher quality data. Systematic spectro-temporal studies of LMXBs using long-term, multi-mission data may give clarity on several unanswered questions that these systems pose.