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We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields which leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain the two-temperature global accretion solutions in terms of dissipation parameters, namely, viscosity (α) and accretion rate (ṁ), and find for the first time in the literature that such solutions may contain standing shock waves. Solutions of this kind are multi-transonic in nature as they simultaneously pass through both inner critical point (x in ) and outer critical point (x out ) before crossing the black hole horizon. We calculate the properties of shock induced global accretion solutions in terms of the flow parameters. We further show that two-temperature shocked accretion flow is not a discrete solution, instead such solution exists for wide range of flow parameters. We identify the effective domain of the parameter space for standing shock and observe that parameter space shrinks as the dissipation is increased. Since the post-shock region is hotter due to the effect of shock compression, it naturally emits hard X-rays and therefore, the two-temperature shocked accretion solution has the potential to explain the spectral properties of the black hole sources.

Relativistic jets and disc-winds are typically observed in BH-XRBs and AGNs. However, many physical details of jet launching and the driving of disc winds from the underlying accretion disc are still not fully understood. In this study, we further investigate the role of the magnetic field strength and structure in launching jets and disc winds. In particular, we explore the connection between jet, wind, and the accretion disc around the central black hole. We perform axisymmetric GRMHD simulations of the accretion-ejection system using adaptive mesh refinement. Essentially, our simulations are initiated with a thin accretion disc in equilibrium. An extensive parametric study by choosing different combinations of magnetic field strength and initial magnetic field inclination is also performed. Our study finds relativistic jets driven by the Blandford & Znajek (BZ) mechanism and the disc-wind driven by the Blandford & Payne (BP) mechanism. We also find that plasmoids are formed due to the reconnection events, and these plasmoids advect with disc-winds. As a result, the tension force due to the poloidal magnetic field is enhanced in the inner part of the accretion disc, resulting in disc truncation and oscillation. These oscillations result in flaring activities in the jet mass flow rates. We find simulation runs with a lower value of the plasma-β, and lower inclination angle parameters are more prone to the formation of plasmoids and subsequent inner disc oscillations. Our models provide a possible template to understand spectral state transition phenomena in BH-XRBs.

We study the relativistic, time-independent, low angular momentum, inviscid, advective accretion flow around Kerr black hole. Considering the relativistic equation of state (REoS), we examine the transonic properties of the flow and find that there exists an upper bound of the location of the physically accepted critical point (r max out ). However, no such limit exists when an ideal gas equation of state (IEoS) is assumed to describe the flow. Further, we calculate the global accretion solutions that contain shock waves and separate the domain of parameter space in angular momentum (λ) and energy (E) plane. We find ample disagreement between the shock parameter spaces obtained for REoS and IEoS, respectively. In general, post-shock flow (equivalently post-shock corona, hereafter PSC) is characterized by shock location (r s ) and compression ratio (R, measure of density compression across the shock front) which are uniquely determined for flow with given input parameters, namely (E, λ). Using r s and R, we empirically compute the oscillation frequency (ν QP O ) of the shock front which is in general quasi-periodic (QP) in nature and retrace the domain of shock parameter space in the r s −R plane in terms of ν QP O for REoS around the weakly as well as rapidly rotating black holes. Finally, we indicate the relevance of the present work to explain the plausible origin of high frequency QPO (HFQPO) and its connection with the spin (a k ) of the Galactic black hole sources.

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