  We study the synchronization of the auto-oscillation signal generated by the
spin transfer driven dynamics of two coupled vortices in a spin-valve
nanopillar to an external source. Phase-locking to the microwave field hrf
occurs in a range larger than 10% of the oscillator frequency for drive
amplitudes of only a few Oersteds. Using synchronization at the double
frequency, the generation linewidth is found to decrease by more than five
orders of magnitude in the phase-locked regime (down to 1 Hz, limited by the
resolution bandwidth of the spectrum analyzer) in comparison to the free
running regime (140 kHz). This perfect phase-locking holds for frequency
detuning as large as 2 MHz, which proves its robustness. We also analyze how
the free running spectral linewidth impacts the main characteristics of the
synchronization regime.
