Radiation Transport Two-Temperature GRMHD Simulations of Warped Accretion Disks

Abstract: In many black hole systems, the accretion disk is expected to be misaligned with respect to the black hole spin axis. If the scale height of the disk is much smaller than the misalignment angle, the spin of the black hole can tear the disk into multiple, independently precessing `sub-disks'. This is most likely to happen during outbursts in black hole X-Ray binaries (BHXRBs) and in active galactic nuclei (AGN) accreting above a few percent of the Eddington limit, because the disk becomes razor-thin. Disk tearing has the potential to explain variability phenomena including quasi-periodic oscillations (QPOs) in BHXRBs and changing-look phenomena in AGN. Here, we present the first radiative two-temperature GRMHD simulation of a strongly tilted ($65^{\circ}$) accretion disk around a $M_{BH}=10M_{\odot}$ black hole, which tears and precesses. This leads to luminosity swings between a few percent and $50 \%$ of the Eddington limit on sub-viscous timescales. Surprisingly, even where the disk is radiation pressure dominated, the accretion disk is thermally stable over $t \gtrsim 21,000 r_g/c$. This suggests warps play an important role in stabilizing the disk against thermal collapse. The disk forms two nozzle shocks perpendicular to the line of nodes where the scale height of the disk decreases $10$-fold and the electron temperature reaches $T_e \sim 10^8-10^9 K$. In addition, optically thin gas crossing the tear between the inner and outer disk gets heated to $T_e \sim 10^8 K$. This suggests that warped disks may emit a Comptonized spectrum that deviates substantially from idealized models.