Institute for Advanced Study Astrophysics Seminar

The X-ray flux of accreting black holes occasionally shows a pair of quasi-periodic oscillations with frequencies in a 3:2 ratio having values comparable to the orbital frequency at the innermost stable circular orbit. The cause and nature of these oscillations is not understood. One proposed explanation is that they are caused by waves in the accretion disk. For a thin disk, the matter can be approximated as a cold collisionless fluid provided that orbits to not intersect. Long-lived perturbations can exist only where the pattern speed is approximately a constant, independent of radius. The resulting waves are analogous to the Lindblad-Kalnajs kinematic density waves in galactic disks. In the case of black holes, while several pairs of modes have frequencies close to a 3:2 ratio, the predicted frequencies are either too high or too low by about a factor of two to explain the observed oscillations. Thus an alternative explanation, perhaps involving parametric resonance, is required to explain the X-ray oscillations of accreting black holes.