Pre-submission Thesis Presentation

Ultraluminous  X-ray sources (ULXs) are one of the brightest X-ray emitting sources with luminosity exceeding the classical Eddington limit of a 10 solar mass black hole (i.e., 10^39 erg/s). We discuss the timing and spectral properties of some individual ULXs and explore some of their fundamental characteristics, like the nature of the compact objects and the physics of accretion flow, to understand how these sources differ from the sub-Eddington X-ray binaries. We discuss the first detection of X-ray flaring events in NGC 4395 ULX1, a source with an intermediate spectral state between soft and supersoft ultraluminous states. The flares are dominated by high-energy photons, suggesting the variability originating from an intrinsic change in accretion rate or due to the presence of variable clumpy wind in the inner region of the accretion disk. We investigate the possible physical origin of the spectral variability and apparent luminosity-power law index anti-correlation in a bright ULX, NGC 1042 ULX1. We present a novel theoretical model based on synchrotron radiation to explain the unique spectral curvature in ULXs. The model suggests that relativistic plasma in a neutron star magnetic field can give rise to such spectral curvature depending on the latitude of the emitted radiation. We also discuss the spectral characteristics of a few other ULX sources, shed light on their underlying super-Eddington accretion mechanism, and set the stage for future directions in ULX science.