Spin-induced disk precession in sagittarius A

In Sgr A* at the Galactic center, by far the closest and easiest supermassive black hole we can study, the observational evidence is increasingly pointing to the presence of a compact, hot, magnetized disk feeding the accretor. In such low Mach number plasmas, forces arising, e.g., from pressure gradients in the plasma, can altogether negate the warping of disks around Kerr black holes caused by the Bardeen-Petterson effect and can lead to coherent precession of the entire disk. In this paper, we present for the first time highly detailed three-dimensional smoothed particle hydrodynamics (SPH) simulations of the accretion disk evolution in Sgr A*, guided by observational constraints on its physical characteristics, and conclude that indeed the Bardeen-Petterson effect is probably absent in this source. Given what we now understand regarding the emission geometry in this object, we suggest that a ∼50-500 days modulation in Sgr A*'s spectrum, arising from the disk precession, could be an important observational signature; perhaps the ∼ 106 days period seen earlier in its radio flux, if confirmed, could be due to this process. On the other hand, if future observations do not confirm this long modulation in Sgr A*'s spectrum, this would be an indication either that the disk size or orientation is very different from current estimates, that the black hole is not spinning at all (unlikely), or that our current understanding of how it produces its radiative output is incorrect.