Spin‐induced Disk Precession in Sagittarius A*

In Sgr A* at the Galactic center, by far the closest and easiest supermassiveblack hole we can study, the observational evidence is increasingly pointing tothe presence of a compact, hot, magnetized disk feeding the accretor. In suchlow-Mach-number plasmas, forces arising, e.g., from pressure gradients in theplasma, can altogether negate the warping of disks around Kerr black holescaused by the Bardeen-Petterson effect and can lead to coherent precession ofthe entire disk. In this Letter, we present for the first time highly detailed3D SPH simulations of the accretion disk evolution in Sgr A*, guided byobservational constraints on its physical characteristics, and conclude thatindeed the Bardeen-Petterson effect is probably absent in this source. Givenwhat we now understand regarding the emission geometry in this object, wesuggest that a ~ 50-500-day modulation in Sgr A*'s spectrum, arising from thedisk precession, could be an important observational signature; perhaps the ~106-day period seen earlier in its radio flux, if confirmed, could be due tothis process. On the other hand, if future observations do not confirm thislong modulation in Sgr A*'s spectrum, this would be an indication that eitherthe disk size or orientation is very different from current estimates, or thatthe black hole is not spinning at all (unlikely), or that our currentunderstanding of how it produces its radiative output is incorrect.Comment: 14 pages, 3 figures; small changes to sections 2.1 and 3; accepted for publication in Ap