| dc.description.abstract | X-ray Binaries (XRBs) are a class of binaries which emit in X-rays. They consist of a compact object, which could be a White Dwarf, Neutron Star or a Black Hole, in orbit with a normal companion star. Black Hole Binaries (BHBs) are XRBs in which the compact object is a Black Hole. BHBs are studied in pursuit of a better understanding of physics in extreme gravity. The physical processes behind the origin of X-ray radiation has been the subject of many studies. Although direct imaging of the sources is next to impossible, spectral and temporal analysis of BHBs can help us ‘see’ the nature and geometry of the sources. can help us ‘see’ the nature and geometry of the sources. Transient BHBs are interesting systems which remain in quiescence for a long period of time but show occasional flaring activity recurring at different timescales. These flares or ‘outbursts’ are often accompanied by changes in both spectral and temporal properties. Over the last few decades, the sources have been found to undergo various phases or ‘states’ in a specific order during an outburst, which are then classified as canonical outbursts. In the present studies, we focus on different sources which do not conform to the accepted picture of the canonical outbursts and perform a comparative study on the nature of the outbursts and the physical processes which drive them. We begin with a brief introduction to different topics like types of X-ray binaries, radiation processes, the evolution of an outburst and the various states associated with them, and so on in Chapter 1. The instruments used to obtain data from these sources and the reduction methods are detailed in Chapter 2. For instruments with a large Field of View (FOV), like Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat, contamination from other sources in the FOV is a challenging issue to be dealt with, while performing spectral analysis. A complete section is dedicated to the method followed to minimize the effects of contamination in such cases towards the end of Chapter 2. We study three such BHB sources in this work - 4U1630-472, MAXI J0637-430 and Swift J1753.5-0127 in subsequent chapters. 4U 1630-472 is a recurrent X-ray binary, which exhibits two different types of outbursts, called ‘mini’ and ‘super’-outbursts (Abe et al., 2005; Capitanio et al.,
2015, etc.). We focus on the 2016 and 2018 ‘mini’-outbursts of the source. The primary instrument used for analysis is the Indian multi-wavelength astronomy satellite AstroSat. The source was initially known to remain in the disc-dominant state throughout the outburst. The initial transition from a low/hard state to an intermediate state is observed for the first time using AstroSat during its
2016 outburst. The transition occurred within a span of ∼ 11 hrs, which was not caught by any pointed instrument previously. The Hardness Intensity Diagram (HID) seems to follow a ‘c’- shaped profile instead of the generally accepted ‘q’ shaped profile observed for BHBs. We also attempt to establish a link between ‘mini’-outbursts and the ‘super’-outbursts, by comparing the HIDs of the
‘mini’-outbursts in 1998 and 1999, and the HID of the ‘super’-outburst of 2002-2004. The spectra are fit using both phenomenological and physical models. Classification into states is performed based on the phenomenological modelling. We also fit the spectra using a two component flow model and comment on the accretion parameters. Mass estimation of the compact object is also obtained from three different methods. This is presented in Chapter 3. In Chapter 4, we present our analysis of the source MAXI J0637-430. It is a relatively new transient source discovered on 2 November 2019, which seems to share some of its properties with the BHB 4U 1630-472. Apparently, this source also remained in the soft state for the most part of the outburst. Similar to 4U 1630-472, no Quasi periodic Oscillations (QPOs) are observed in thePower Density Spectra (PDS). As with the source 4U 1630-472, we perform spectral fits using phenomenological and physical models and try to divide the outburst into different states. We also obtain mass estimates using different methods. We also try to establish a possible link between the two sources by studying the individual HID patterns. Finally, we comment on the underlying physical mechanisms which could possibly drive the two sources. In Chapter 5, we move on to the source Swift J1753.5-0127, which remained in the hard state for most of the outburst (Ramadevi and Seetha, 2007; Zhang et al., 2007). This is diametrically opposite to the two sources studied above. Prominent QPOs are observed in the PDS and the duration of the outburst is much longer than that observed for the other two sources. Here, we adopt a different approach and try to comment on the accretion geometry using Frequency Resolved Spectroscopy (FRS).In Chapter 6, we present a summary of our results and comment on the future studies based on the obtained results. Recommendations based on this work are listed in Chapter 7. | en_US |
