  String loop vibrations in a central plane of a Schwarzschild black hole are
investigated for various string equations of state. We discuss string loop
stability and derive frequencies of vibrational modes. Using the vibrating
string loop model we fit the quasi-periodic oscillation (QPO) observed in X-ray
signal coming from some compact sources. We demonstrate how the string-loop
parameters are related to the radial and vertical fundamental vibration modes,
and how the vibrational instability can be related to the Q-factor
characterizing the observed QPOs.
