Understanding Extreme Accretion and Outflows Around Black Holes through Radiative Transfer Studies

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Abstract: This dissertation investigates the observational characteristics of super-Eddington accretion and outflow through advanced radiative transfer simulations. In the first part of the dissertation, I developed and utilised a general relativistic ray-tracing package to examine the X-ray reverberation signatures from super-Eddington accretion flows. My study reveals that the spectral and temporal signatures of Fe lines generated by super-Eddington flows have distinct morphologies compared to those produced by thin accretion disks: the line energy is more blueshifted, the line profiles are more symmetric, and the time lag of the line is much shorter. I apply my calculations to successfully model the Fe line observed from the jetted tidal disruption event Swift J1644+57, and I give direct quantitative evidence that a super-Eddington disk has likely formed in this event.

In the second part of the dissertation, I employ a state-of-the-art Monte Carlo Radiative Transfer code, Sedona, to calculate the continuum emission produced from simulated super-Eddington accretion flows, focusing on their relevance to tidal disruption events. I find that the spectral energy distribution depends not only on the inclination of the observer with respect to the disk but also on the accretion rate. Therefore, my results can be used to explain the diversity and evolution of TDEs, in which the accretion rate is expected to change rapidly over a timescale of years. In particular, I apply this model to explain tidal disruption events that start as optically bright but evolve to be X-ray bright later, such as ASASSN 15-oi.

This dissertation emphasises the importance of using radiative transfer calculations to study and understand super-Eddington accretion and outflow around black holes. The findings provide valuable insights into the physical processes involved in these events and contribute to developing more accurate models for future observations.