SIMBAD references

There is some evidence, although yet unconfirmed, that Sagittarius A*–the supermassive black hole at the Galactic center–emits its radio waves modulated with a ∼100 day period. What is intriguing about this apparent quasi periodicity is that although the amplitude of the modulation increases with decreasing wavelength (from 3.6 to 1.3 cm), the quasi period itself does not seem to depend on the frequency of the radiation. It is difficult to imagine how a binary companion, were that the cause of this modulation, could have escaped detection until now. Instead, it has been suggested that the spin-induced precession of a disk surrounding a slowly rotating black hole could have the right period to account for this behavior. In this paper, we examine how Sgr 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-dependent amplitude. This appears to be in line with other observational arguments suggesting that Sgr A*'s millimeter/submillimeter spectrum is produced by a ∼10 Schwarzschild radius disk, whereas its centimeter waves originate from a nonthermal particle distribution in a halo extending out to over 20 Schwarzschild radii. Interestingly, this model suggests that the observed period corresponds to half the precession period and that a nonaxisymmetric disk could produce a second period roughly twice as long as the first.