|Abstract : ||Black hole (BH) accretions are very powerful sources of energy in the universe. In accretion phenomena, the surrounding gas spirals down towards the central BH and forms a disc or quasi-spherical structure based on the outward transport of angular momentum. At the same time, the enormous gravitational potential energy of a BH is released as heat and radiation. However, the presence of strong magnetic fields can alter the underlying radiation mechanisms and thermodynamic properties. Theoretical and observational inferences indicate a signature of dynamically dominant magnetic fields in the vicinity of BHs. This thesis work is on understanding the role of large-scale strong magnetic fields on optically thin and advective accretion flows in which magnetic fields influence the accretion dynamics, remove angular momentum from in-falling matter, help in the formation of strong outflows, and enhance the cooling mechanism through synchrotron and synchrotron self-Comptonization processes.
We have explained the decelerating nature of accreting matter, and the origin of different magnetic barriers near the central BH in the context of Magnetically Arrested Advective Accretion Flows (MAAAFs). As an immediate observational consequence, we have shown, for the first time, that our MAAAF model around rapidly spinning stellar-mass BHs can explain some long-standing issues of hard-state ultraluminous X-ray sources, which are very bright, point sources with apparent luminosities exceeding the standard Eddington limit for a stellar-mass BH. We suggest that there is no need to incorporate the contentious intermediate-mass BH scenarios nor super-Eddington accretions. We have also addressed a unified classification of blazars, a particular class of active galactic nucleus, by solving magnetized disc-outflow symbiosis self-consistently and compared with Fermi blazar observations.
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