A Precessing Ring Model for Low‐Frequency Quasi‐periodic Oscillations

We develop a simple physical model to describe the most common type oflow-frequency quasi-periodic oscillations (QPOs) seen in a number of accretingblack hole systems, as well as the shape of the relativistically broadened ironemission lines that often appear simultaneously in such sources. The model isbased on an inclined ring of hot gas that orbits the black hole along geodesictrajectories. For spinning black holes, this ring will precess around the spinaxis of the black hole at the Lense-Thirring (``frame-dragging'') frequency.Using a relativistic ray-tracing code, we calculate X-ray light curves andobserved energy spectra as a function of the radius and tilt angle of the ring,the spin magnitude, and the inclination of the black hole. The model predictshigher-amplitude QPOs for systems with high inclinations, as seen in a growingnumber of black hole binary systems. We find that the Rossi X-ray TimingExplorer observations of low-frequency QPOs in GRS 1915+105 are consistent witha ring of radius R ~ 10M orbiting a black hole with spin a/M ~0.5 andinclination angle of i ~ 70 deg. Finally, we describe how future X-ray missionsmay be able to use simultaneous timing and spectroscopic observations tomeasure the black hole spin and probe the inner-most regions of the accretiondisk.Comment: Accepted to ApJ, 22 pages, 7 figure