Nodding motions of accretion rings and disks - A short-term period in SS 433

Abstract

We present a formalism to compute frequencies and amplitudes of short-term "nodding" motions in precessing accretion disks in close binary systems. This motion occurs when the binary orbital period is not negligible compared with the precession period, as is the case, for example, in both SS " 433 and Hercules X-l. The analysis predicts a 6f3 periodic component of the "moving" spectral Unes of SS 433, at an amplitude ~ 10% of the 164 d precessional motion. Examination of spectra of SS 433 obtained on more than 300 nights during a 3 year period in fact reveals the expected period at approximately the correct amplitude. Further, our analysis argues strongly that the 164 d period is a slaved rather than a driven precession, and that the disk has remarkably high viscosity and radius of the order of that of the Roche lobe. Observations of this nodding effect in SS 433 also provide an accurate determination of the binary orbital period (13 d 086±0 d 004) and a stringent Emit on any change in this period, |dP{\y)/dt \ <5X10-5. Some applications of the formalism to Hercules X-l are also briefly considered; in particular, an explanation is suggested for the curious phenomenon that the 35 d X-ray "turn-ons" occur at two specific phases of the l d 7 binary period.