Discovery of the upper kilohertz quasi-periodic oscillation from the X-ray transient Aql X-1

We report on a comprehensive analysis of the kilohertz (>= 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 1-kHz QPO has been reported, whereas in most sources, 2-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 2-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 the upper QPOs relates to the neutron star spin frequency is still as good as any comparably simple model.