Numerical modeling of accretion flow around black hole: study of statistics and energetics of PLASMOIDS

Abstract

Flares from close proximity of black hole neighborhood (in AGN and BH-XRBs) and the orbital motion of a luminous blob around the galactic center have recently been ob served, indicating rich and complex magnetohydrodynamics in the core accretion zone (tens of gravitational radii). Light curves of such sources with rapid variability appear to have a time scale that is substantially shorter than the event horizon’s light-crossing time scale. As a result of the causality condition, this time scale corresponds to a few Schwarzschild radii of the emitting region’s length scales. The observations from such regions can be consistently explained by the non-thermal radiation from the plasmoids both in terms of time scale and energetics. Plasmoids are blobs of plasma carrying highly energetic particles formed in the magnetic reconnection process. In the finite physical resistivity regime, the magnetic reconnection process is an efficient method for convert ing magnetic energy into thermal dissipation and kinetic energy of the particles in the accreting fluid. In order to investigate the dynamics and evolution of such system, In the framework of General Relativistic Magnetohydrodynamics (GRMHD), a resistive ax isymmetric geometrically thin accretion disc in hydrodynamic equilibrium is numerically set up around a maximally spinning Kerr black hole. Numerical simulations in GRMHD framework are performed with the thin resistive disk up to a time of t = 2000GM/c3 , taking two distinct ohmic resistivity values, moderate and low. For the mildly resistive disk, we investigate the size distribution and energetics of the resultant plasmoids that are formed in the black hole magnetosphere region. The generation and size distribution of plasmoids, for which the substantial kinetic energy released in reconnection events in the region with high magnetization outweighs the gravitational potential energy of the black hole, are examined. For detecting outgoing plasmoids, we employ the Bernoulli parameter distribution. The plasmoids are located in the 2D simulation snapshot us ing technique of binary opening as opposed to the conventional technique of detecting X-points and O-points. A different study on the size distribution of plasmoids formed via explosive magnetic reconnection in a simple system of double current sheet is also carried out in resistive MHD framework to validate the detection technique. We obtain reasonable results from the study which are physically significant. We also address the plasmoids’ energy outputs quantitatively, which could account for later flaring activity in AGNs. We use a very simple model to do so, assuming that a fraction of the ohmic heat energy contributes to non-thermal plasmoid emission. A number of free parameters associated with the energy relation are explored against range of different values to ob tain the optimum ones. Finally, we show the non-thermal energy output, converted into suitable physical units, does in fact matches with observation of energy output of AGN flares.