| Abstract : | Jets are observed in the low/hard state of a transient black hole binary (BHB). Jet production is proposed to be connected to the presence of a radiatively inefficient accretion flow (RIAF) and the large-scale magnetic fields close to the accreting black hole (BH). However, the source of the large-scale fields is not quite obvious. Two plausible sources of the large-scale magnetic fields are: i) in-situ generation of the magnetic field by dynamo action and ii) advection of the large-scale field from an external source (e.g., the companion star in BHBs). We studied the efficiency of both processes using state of the art 3D global magnetohydrodynamic (MHD) and general relativistic magnetohydrodynamic (GRMHD) simulations respectively. First, using MHD simulations, we studied the large-scale magnetic field generation by a magnetorotational instability (MRI) driven dynamo in a geometrically thick RIAF. We used the mean-field paradigm to describe the dynamo action and characterize dynamo coefficients for the first time in global simulations of the accretion flows. We found evidence of a weak α-effect and a strong turbulent pumping leading to an inefficient generation of the ordered large-scale magnetic field in a RIAF. Next, we studied the plausibility of accumulation of large-scale fields due to the accretion of external magnetic field loops of different sizes and strengths in a turbulent quasi-stationary weakly magnetized RIAF using GRMHD simulations. We found that not all the flux injected at the outer radii can reach the BH; rather, a certain fraction of it does (15 -50 percent). Relatively higher efficiency of flux transport observed in our study hints at the plausible formation of a magnetically dominated RIAF close to the black hole (and hence jets), provided that a source of the large-scale field exists at the larger radii. |
