MG13 - Talk detail

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We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We show that in presence of an axisymmetric soft-photon source the spherically symmetric Bondi flow loses its symmetry and becomes axisymmetric but non-spherical. Using the Monte Carlo method, we generated the radiated spectra as functions of the accretion rates. We find that the transitions from a hard state to a soft state is determined by the mass accretion rates of the disc and the halo. We separate out the signature of the bulk motion Comptonization and discuss its significance. We study how the net spectrum is contributed by photons suffering different number of scatterings and spending different amounts of time inside the Compton cloud. We study the directional dependence of the emitted spectrum as well. The accretion flow with angular momentum was observed to slow down close to the axis and formed a shock or boundary layer. This layer is produced primarily due to the centrifugal barrier that the accretion flow experiences. We keep the inner edge of the Keplerian disk at the shock location when we carry out the time-dependent coupled hydrodynamics and the radiative transfer simulation. We find that Quasi-periodic oscillations (QPOs) are primarily due to the oscillations of this layer. We show that the observational quantities like spectral slope, hard photon count etc. give the same QPO frequencies as the frequency with which the shock location or the boundary layer is oscillating.

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