Radiation-driven Origin of Dynamically Significant Magnetic Fields around Accretion Disks

Magnetic fields play a central role in accretion physics around black holes (BHs), yet their physical origin within accretion flows remains an open problem. We investigate the generation and subsequent evolution of magnetic fields triggered by anisotropic radiation fields in BH accretion disks with compact rotating inner corona. We evolve the magnetic field using the generalized kinematic magnetic field evolution, including induction and Hall effects. The radiation field acts as a primary field generator, while azimuthal rotation in the magnetized plasma provides rapid amplification leading to magnetic-field strengths of order ∼10⁸ G in the vicinity of a 10 M_⊙ BH, and accretion disk and corona emitting at Eddington limit. We show that this process is very fast: the generated magnetic fields exceed local equipartition estimates based on gas pressure within a viscous time scale. When vertical outflows are present, the amplified magnetic fields are advected outward. Our results demonstrate that radiation is not merely a passive component of accretion flows, but provides a robust and unavoidable trigger for the generation of dynamically significant magnetic fields. Furthermore, they provide a physically motivated pathway for the origin of large-scale, structured magnetic fields in and around accretion disks, without requiring magnetorotational-instability-driven turbulent amplification or externally advected magnetic flux. This mechanism offers a pathway toward magnetization of accretion disks and surrounding outflowing plasma without invoking externally supplied magnetic flux, and has broad implications for X-ray binaries, active galactic nuclei, and other transients such as gamma-ray bursts.