The multiwavelength spectral and timing nature of the microquasar Cygnus X-3

Microquasars are double star systems with a compact object, i.e. a neutron star or a black hole, accreting matter from a companion star and occasionally channeling the accreted matter into highly relativistic particle beams called jets. Amongst microquasars Cygnus X-3 is one of the most luminous and as such is an ideal source to study the connection between gravitationally heated, accreted matter close to the compact object and the jet. Cygnus X-3 is also a unique source among microquasars as it contains a massive companion star that drives a strong stellar wind, which causes additional emission scenarios when interacting with the compact object and the jet. This thesis sets out to search and study the observational phenomena caused by this special set of conditions, and attempts to resolve what the link is between different emission scenarios during violent outbursts - the birth of the jet - observed from the source. To study the disk-jet connection in detail a hardness-intensity diagram (HID) is constructed from broadband X-ray data with simultaneous radio observations. Based on the HID a new X-ray/radio state is identified in this thesis: the hypersoft state. This state is softer in the X-ray spectra than previously classified states and exhibits very weak or nonexistent radio emission. The hypersoft state is found to be connected to the jet ejection episodes and to gamma-ray emission detected from Cygnus X-3. The fact that the sequence of events giving birth to a jet differs from other microquasars could be attributed to the strong stellar wind component and/or our line-of-sight almost coinciding with the jet axis. In addition, quasi-periodic oscillations (QPOs) are searched for in the noise-rich X-ray light-curves in an effort to pinpoint the environment in which these oscillations occur. The search resulted in two detections, which are associated to a certain extent with jet ejection episodes. That the QPOs coincide with outbursts points to a direct or indirect jet origin of the QPOs. Finally, principal component analysis (PCA) is used to study the variability of X-ray emission components during outbursts. The PCA studies demonstrate a method for eliminating spectral-fit degeneracy that the X-ray community should be enticed into embracing more widely. The PCA showed that there are two main variability components in play during outbursts. In addition, a double soft-seed population Comptonization scenario is proposed that might occur in other microquasars as well, where so far mostly single soft-seed photon Comptonization models have been used. Multiple studies have shown that such a hot component is required, supporting a different source for the seed photons besides the disk.