Radiation-Driven Origin of Super-Equipartition Magnetic Fields in Accretion Discs and Outflows

Abstract

Magnetic fields play a central role in accretion physics around black holes, yet their physical origin within accretion flows remains an open problem. In this work, we investigate the generation and subsequent evolution of magnetic fields triggered by anisotropic radiation fields in black hole accretion discs with compact rotating inner corona. We self-consistently evolve the magnetic field using the generalized field evolution MHD equation, including advection, shear-driven induction, and Hall effects. The radiation field acts as a primary field generator, while azimuthal rotation in the magnetized plasma provides rapid amplification. We find that radiation-generated fields efficiently reach a dominant toroidal component by Keplerian rotation, leading to magnetic field strengths of order $\sim 10^{8}\,\mathrm{G}$ in the vicinity of a 10 solar mass black hole and accretion disc-corona emitting at luminosity equivalent to the Eddington unit. These magnetic fields are achieved within viscous timescales and reach or exceed local equipartition estimates based on gas pressure. When vertical outflows are included, the amplified magnetic fields are advected into the corona, magnetizing disc-launched winds and jet precursors with field strengths of similar order. 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. Our results provide a physically grounded explanation for the origin of large-scale, structured magnetic fields in and around accretion discs. This mechanism offers a pathway for magnetizing accretion discs and their outflows without invoking externally supplied magnetic flux, with broad implications for X-ray binaries, active galactic nuclei and other transients such as gamma-ray bursts (GRBs).

Figures (5)

• Figure 1: Schematic illustration of the adopted disc--corona geometry. A geometrically thin Keplerian disc extends radially from $x_c$ to $x_0$ situated at $z = 0$, while a compact, opaque corona occupies an oblique region of radius $x_c$ and vertical extent $z_m$ above the black hole. The corona emits isotropically and is treated as optically thick, so radiation entering its interior is absorbed. This geometry is used to compute the anisotropic radiation field and the resulting radiation--driven magnetic source term as shown in the appendix.
• Figure 2: Time evolution of the magnetic field components for a vertically outflowing plasma following the velocity profile given by equation \ref{['eq_vz']}. The three columns correspond to times $t=0.2$, $0.6$, and $1.0\,\mathrm{s}$ (advective timescale). The top three rows show the radial ($B_r$), azimuthal ($B_\phi$), and vertical ($B_z$) components, while the bottom row shows the magnetic field magnitude $|\mathbf{B}|$. The radiation source term $\mathbf{F}_0(r,z)$ is obtained from precomputed radiative transfer tables for a luminous disc--corona system around a $10\,M_\odot$ black hole. Magnetic fields are initialized to zero and evolve under the combined action of radiation forcing, shear-driven induction, and the Hall term. The polarity reversal of magnetic fields above $r=3r_g$, especially in the azimuthal component, is caused by the interplay between the radiation term and the induction term. The black dotted line marks the geometrical extension of the corona.
• Figure 3: Same as Figure \ref{['lab_Gen_res']} but with $v_z=0$, which implies the magnetization of the accretion disc upper surface due to disc-corona radiation field. In absence of vertical velocity component, the magnetization in plasma remains confined to the region close to the disc plane. The maximum runtime $1$ second corresponds to the typical viscous timescale.
• Figure 4: The evolution of maximum magnetic field components for magnetized outflows in Figure \ref{['lab_Gen_res']} (top panel) and magnetized disc surface in Figure \ref{['lab_Gen_res_disk']} (bottom panel)
• Figure 5: Variation of Maximum magnitudes of magnetic field as shown in Figure \ref{['lab_max_mag_from_combined_files']} at $t=0$ for disc outflows (blue solid) and disc surface (dashed magenta) with corona size (top panel) and corona luminosity (bottom panel). The obtained relations are $B_{max}\propto 1/x_c^2$ and $B_{max}\propto L_c$