Discovery of the upper kilo-Hz QPO from the X-ray transient Aql X-1

摘要

We report on a comprehensive analysis of the kilo-Hz (>~600 Hz)quasi-periodic oscillations (kHz QPOs) detected from the neutron star X-raytransient Aquila X-1 (Aql X-1) with the Rossi X-ray Timing Explorer, between1997 and 2007. Among kHz QPO sources, Aql X-1 is peculiar because so far onlyone kHz QPO has been reported, whereas in most sources, two kHz QPOs areusually detected (a lower and an upper kHz QPO). The identification of the QPOsreported so far has therefore been ambiguous, although it has been proposedthat they were likely to be the lower QPO. Following up on previous work, weconfirm the identification of the QPOs previously reported as lower QPOs,because of their high quality factors and the quality factor versus frequencydependency, which are similar to those observed in other sources. Combining allsegments of data containing a lower QPO, we detect for the first time an upperkHz QPO. As in other sources for which the neutron star spin frequency islarger than 400 Hz (550.25 Hz in Aql X-1), the frequency difference between thetwo kHz QPOs is close to half the spin frequency. Based on this result, were-examine the link between the neutron star spin and the frequency of the kHzQPOs, to show that a model in which the separation of the lower and upper QPOsrelates to the neutron star spin frequency is still as good as any comparablysimple model.