  We study the behavior of the linewidth of a nano-contact based spin torque
oscillator (STO) under application of a radio frequency (100 MHz) modulating
current. We achieve a significant (up to 85%) reduction of the STO linewidth
when it is modulated across a region of high nonlinearity. The mechanism
responsible for the linewidth reduction is the nonlinear frequency shift under
the influence of current modulation, which reduces the nonlinear amplification
of the linewidth. The reduction of the linewidth during modulation can be
quantitatively calculated from the free-running behavior of the STO.
