Observational Signatures of Tilted Black Hole Accretion Disks

We have performed time-dependent radiative transfer on data from global, 3D, general relativistic MHD simulations of a tilted black hole accretion flow from the code Cosmos++. The flow is tilted in that the black hole spin axis is misaligned from the angular momentum axis of the accreting torus. Using Geokerr, we compute photon orbits in a Kerr spacetime for a grid of rays, and solve the radiative transfer equation for desired emission and absorption coefficients in the region where the rays intersect the accretion flow. This creates a coordinate-independent observed image of the black hole. Repeating the procedure at many at many simulation time steps produces movies.

Here are some movies from toy emission models.

Movies of optically thin line emission at two observer inclinations. The panel size is 50x40M for i=60 degrees and 50x50M for i=0.

[60 degrees] [0 degrees (face on)]

Here are the equivalent movies of the untilted simulation.

[60 degrees] [0 degrees (face on)]

Movies of the rho*T emissivity at two observer inclinations. The panel size is the same as before.

[60 degrees] [0 degrees (face on)]

Here are the equivalent movies of the untilted simulation.

[60 degrees] [0 degrees (face on)]

The inner edge of a tilted disk does not move inward with increasing black hole spin as is the case for standard disks. We can see this by computing a sequence of images that exclude a sphere of fluid with varying radius. As the radius gets bigger, the total flux emitted drops. The "radiation edge" is the radius of the sphere where the total flux drops below some fraction of its maximum. As shown in the following plots, the radiation edge for the untilted simulations follows the expected trend. The radiation edge for the tilted simulations is seemingly independent of black hole spin.

Plots of relative flux for increasing radius of the excluded region, and of the radiation edge vs. spin for all simulations from the thin line emissivity. Simulations with names ending in "15h" are tilted, while those ending in "0h" are untilted. In the second plot, the open (filled) circles are for the untilted (tilted) simulations

[Relative intensity vs. radius of cut out sphere] [Radiation edge vs. spin]

Emission lines in accretion disks are smeared out due to Doppler boosting and gravitational redshifts. In standard, untilted disks the line profile has a red wing from gravitational redshifts and a sharp blue peak from Doppler beaming. The tilted disks can produce a broad blue wing as well. The width of the red wing in untilted disks is related to the inner edge of the disk. Since the inner edge of the tilted disks is roughly independent of spin, so too is the width of their red wings. Finally, the shape of the emission line profile changes significantly with the observer's azimuth. Since tilted black hole accretion flows are expected to precess, the line profiles should significantly vary with time.

Plots of emission line shapes for untilted simulation (left panel) and four observer azimuths of tilted simulation.

[Line shapes at 60 degree inclination]

Reference: Dexter, J. & Fragile, P. C., ApJ, 730, 36.