Investigating the emission properties and the geometry of black hole X-ray binaries in the outburst cycle

Depending on their initial mass, stars can live from millions up to billions of years. This mass also determines the next step in their stellar evolution when they ran out of their fuel. The cores of stars with a mass greater than 20 M collapse into a black hole (BH). This object does not produce any radiation by itself, however, if it is in a binary system, the companion star may supply matter that accretes onto the BH. The angular momentum of the accreted material prevents a direct fall and matter forms a disk around the BH. The radiation from this accretion disk is observable. Under certain circumstances, a portion of this disk thought to evaporate by the effect of heat conduction and turns into a radiatively inefficient hot-flow. The relativistic electrons in the flow Compton-up scatter the disk photons and produce hard X-rays. The material in the disk may also be ejected away on some region close the BH with the help of strong magnetic field which produces the jets observed in radio and near-infrared wavelengths. These phenomena have a tendency to repeat themselves in a hysteresis-like loop. Constant monitoring of BH X-ray binaries provided a classification based on the abrupt changes in spectral and temporal properties of the sources. According to this classification, BHXBs spend most of their time in a low flux state known as quiescence. At the very beginning of the outburst, the system goes into a hard state in which the spectrum is dominated by high energy photons and variability increases. The compact jets are also observed in this state. As the material starts to move radially inward, the luminosity increases and variability drops. This state is known as the soft state. A disk wind may be observed in this state for high inclination sources. At the end of the outburst where mass accretion rate ( Ṁ) is sufficiently low, evaporation may remove majority of the disk which cause the source returns back into its initial state. There are also intermediate states which show properties of both hard and soft states with unique features. Unfortunately, there is no universal model that explains all the dynamic properties of BHXBs in different spectral states. Some models take into account GR effects and others include dynamo process as well as change in accretion rate. Interpretation of these models allows us to understand the motivation behind the observed phenomena. For instance, jets are events which usually are observed in hard and hard-intermediate states in a timescale of hours to months. Although the mechanism behind this phenomena is still debated multi-wavelength observations put forward a strong correlation between radio and X-ray luminosity indicating a disk-jet coupling. This nonlinear relation between inner flow and outflow (X-ray-radio luminosity) is thought to be simple and unique at first. Yet, along with the increase in the amount of data, it became clear that numerous BH systems are "outliers" follow a steeper path in radio X-ray correlation. Several models have been proposed to explain this strange behavior. One of the most promising hypothesis is the condensation disk model where it is assumed that a cooler weak disk is reformed at the end of the outburst which provides the additional X-ray flux. As Ṁ decreases the disk can no longer re-condense and disperse. The outliers which joins the main track below a certain flux value are considered to experience such a process. This hypothesis can be tested by determining the geometry of the system since the corona (hot-flow) must retrieve at some point in order to allow the inner disk to reform. The first part of this thesis is devoted to testing this hypothesis by modeling the power spectrum of both regular and outlier sources with a new developed model PROPFLUC. This is a physical model based on the mass accretion rate fluctuations in the precessing corona. Temporal analyses of sources with such a model allowed us to extract the inner and outer radius of the corona. By observing the evolution of these parameters we aim to understand how disk and corona behave relative to each other during outburst decay and rise. In this part, data from 4 different BH transients (GX339-4, 4U1543-47, GRO J1655-40, XTEJ1550-564) have been analyzed in hard and hard-intermediate states for multiple outbursts between 2000-2011. Raw data were obtained from the Rossi X-ray Timing Explorer (RXTE) and processed according to the procedure stated in the Guest Observer Facility of NASA-HEASARC. Analyses are generally done in both XSPEC and IDL programs and additional bash and TCL scripts were used to automate the processes. In the outburst cycle, the transition from hard to soft state occurs at higher luminosity compared to transition from soft to the hard state. The second part of this thesis covers the spectral and timing analyses of BHXBs in outburst decay in order to obtain the luminosity distribution of these sources. By exploring the key parameters of state transition, we aim to investigate the origin of hysteresis in the hardness-intensity diagram as well as investigating the variability of state transition luminosities for multiple sources in the outburst decay. The transition luminosities were found for 2 separate classifications in the literature namely Belloni and Kalemci classifications. The analyses were done for 12 BH transients in 20 outburst decays. The spectral fits were mainly done via tbabs*smedge*(powerlaw+diskbb) model. In addition, Gauss and hecut models also tested in order to include the effect of the iron line (emission around 6.4 keV) and the exponential cut-off in the power-law predicted by thermal Componization models. The timing part was mainly modeled with multi Lorentzians where broad-band noise and QPO corresponds to broad and narrow Lorentzians respectively.