Long‐Term Radio Modulation in Sagittarius A* from Spin‐induced Disk Precession

There is some evidence, though yet unconfirmed, that Sagittarius A*--thesupermassive black hole at the Galactic center--emits its radio waves modulatedwith a ~100-day period. What is intriguing about this apparentquasi-periodicity is that, though the amplitude of the modulation increaseswith decreasing wavelength (from 3.6 to 1.3 cm), the quasi-period itself doesnot seem to depend on the frequency of the radiation. It is difficult toimagine how a binary companion, were that the cause of this modulation, couldhave escaped detection until now. Instead, it has been suggested that thespin-induced precession of a disk surrounding a slowly rotating black holecould have the right period to account for this behavior. In this paper, weexamine how Sagittarius A*'s light curve could be modulated by this mechanism.We demonstrate that the partial occultation of a nonthermal halo by a compact,radio-opaque disk does indeed produce the observed frequency-dependentamplitude. This appears to be in line with other observational argumentssuggesting that Sagittarius A*'s mm/sub-mm spectrum is produced by a ~10Schwarzschild-radius disk, whereas its cm-waves originate from a nonthermalparticle distribution in a halo extending out to over 20 Schwarzschild radii.Interestingly, this model suggests that the observed period corresponds to halfthe precession period and that a non-axisymmetric disk could produce a secondperiod roughly twice as long as the first.Comment: 17 pages, 5 figures, accepted to Ap