The 67 Hz Feature in the Black Hole Candidate GRS 1915+105 as a Possible “Diskoseismic” Mode

The Rossi X-Ray Timing Explorer has made feasible for the first time the search for high-frequency (≳100 Hz) periodic features in black hole candidate (BHC) systems. Such a feature, with a 67 Hz frequency, recently has been discovered in the BHC GRS 1915+105 (Morgan, Remillard, & Greiner). This feature is weak (rms variability ~0.3%-1.6%), stable in frequency (to within ~2 Hz) despite appreciable luminosity fluctuations, and narrow (quality factor Q ~ 20). Several of these properties are what one expects for a “diskoseismic” g-mode in an accretion disk about a 10.6 M☉ (nonrotating) to 36.3 M☉ (maximally rotating) black hole (if we are observing the fundamental-mode frequency). We explore this possibility by considering the expected luminosity modulation, as well as possible excitation and growth mechanisms—including turbulent excitation, damping, and “negative” radiation damping. We conclude that a diskoseismic interpretation of the observations is viable.