  A theoretical formula of the linewidth caused by the thermal activation in a
spin torque oscillator with a perpendicularly magnetized free layer and an
in-plane magnetized pinned layer was developed by solving the stochastic
Landau-Lifshitz-Gilbert equation in the energy-phase representation. It is
shown that the linewidth can be suppressed down to 0.1 MHz by applying a large
current (10 mA for typical material parameters). A quality factor larger than
10^{4} is predicted in the large current limit, which is two orders of
magnitude larger than the recently observed experimental value.
