  We report on a comprehensive analysis of the kilo-Hz (>~600 Hz)
quasi-periodic oscillations (kHz QPOs) detected from the neutron star X-ray
transient Aquila X-1 (Aql X-1) with the Rossi X-ray Timing Explorer, between
1997 and 2007. Among kHz QPO sources, Aql X-1 is peculiar because so far only
one kHz QPO has been reported, whereas in most sources, two kHz QPOs are
usually detected (a lower and an upper kHz QPO). The identification of the QPOs
reported so far has therefore been ambiguous, although it has been proposed
that they were likely to be the lower QPO. Following up on previous work, we
confirm the identification of the QPOs previously reported as lower QPOs,
because of their high quality factors and the quality factor versus frequency
dependency, which are similar to those observed in other sources. Combining all
segments of data containing a lower QPO, we detect for the first time an upper
kHz QPO. As in other sources for which the neutron star spin frequency is
larger than 400 Hz (550.25 Hz in Aql X-1), the frequency difference between the
two kHz QPOs is close to half the spin frequency. Based on this result, we
re-examine the link between the neutron star spin and the frequency of the kHz
QPOs, to show that a model in which the separation of the lower and upper QPOs
relates to the neutron star spin frequency is still as good as any comparably
simple model.
