Electromagnetic vs. Lense-Thirring alignment of black hole accretion discs

Abstract

Accretion discs and black holes (BHs) have angular momenta that are generally misaligned with respect to each other, which can lead to warps in the discs and bends in any jets produced. We consider a disc that is misaligned at large radii and torqued by Lense-Thirring (LT) precession and a Blandford-Znajek (BZ) jet torque. We consider a variety of disc states that include radiatively inefficient thick discs, radiatively efficient thin discs, and super-Eddington accretion discs. The magnetic field strength of the BZ jet is chosen as either from standard equipartition arguments or from magnetically arrested disc (MAD) simulations. We show that standard thin accretion discs can reach spin-disc alignment out to large radii long before LT would play a role, as caused by the slow infall time that gives even a weak BZ jet time to align the disc. We show that geometrically thick radiatively inefficient discs and super-Eddington discs in the MAD state reach disc-spin alignment near the black hole when density profiles are shallow as in magnetohydrodynamical simulations, while the BZ jet aligns discs with steep density profiles (as in advection-dominated accretion flows) with the BH spin out to larger radii. Our results imply that the BZ jet torque should affect the cosmological evolution of BH spin magnitude and direction, BH spin measurements in active galactic nuclei and X-ray binaries, and the interpretations for Event Horizon Telescope observations of discs or jets in strong-field gravity regimes.

11 pages, 3 figures, submitted to MNRAS

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Cited by in corpus (8)

• Formation of Precessing Jets by Tilted Black-hole Discs in 3D General Relativistic MHD Simulations
• Electromagnetic Counterparts to Massive Black Hole Mergers
• Observational signatures of disk and jet misalignment in images of accreting black holes
• Tilted Disks around Black Holes: A Numerical Parameter Survey for Spin and Inclination Angle
• The Physics of Accretion Discs, Winds And Jets in Tidal Disruption Events
• Misaligned magnetized accretion flows onto spinning black holes: Magneto-spin alignment, outflow power, and intermittent jets
• Tilted Circular Orbits around a Kerr Black Hole
• Fast 3C 279 gamma flares by a merging medium size black hole jet aligned to the AGN one by tidal torque?