Discovery of the upper kilo-Hz QPO from the X-ray transient Aql X-1
Didier Barret (CESR, Toulouse), Martin Boutelier (CESR, Toulouse), M., Coleman Miller (Univ. of Maryland)
TL;DR
This paper reports the first detection of an upper kHz QPO from Aql X-1, clarifying the relationship between QPOs and neutron star spin, and supporting models linking QPO separation to spin frequency.
Contribution
The study confirms the identification of previously reported lower QPOs and for the first time detects an upper QPO in Aql X-1, enhancing understanding of QPO characteristics.
Findings
Detection of the first upper kHz QPO in Aql X-1.
Frequency difference between QPOs is close to half the neutron star's spin frequency.
Supports models linking QPO separation to neutron star spin.
Abstract
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…
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