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

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 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.

arXiv commentAccepted for publication in MNRAS, 7 pages, 4 figures, 4 Tables. Updated reference to Strohmayer, Markwardt, Kuulkers (2007, ApJL, in press) for the discovery of the spin frequency in 4U0614+09

References in corpus (28)

• Sonic-Point Model of Kilohertz Quasi-Periodic Brightness Oscillations in Low-Mass X-ray Binaries
• Correlations in Quasi-Periodic Oscillation and Noise Frequencies Among Neutron-Star and Black-Hole X-ray Binaries
• Discovery of coherent millisecond X-ray pulsations in Aql X-1
• Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
• Evidence for a 2:3 resonance in Sco X-1 kHz QPOs
• An abrupt drop in the coherence of the lower kilohertz QPO in 4U 1636-536
• Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34
• Kilohertz Quasi-Periodic Oscillation Peak Separation is not Constant in the Atoll Source 4U 1608-52
• Discovery of a second kHz QPO peak in 4U 1608-52
• Milli-second Oscillations in the Persistent and Bursting Flux of Aql X-1 During an Outburst
• The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars
• The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
• Changing Frequency Separation of Kilohertz Quasi-Periodic Oscillations in the Sonic-Point Beat-Frequency Model
• Discovery of Two Simultaneous Kilohertz Quasi-Periodic Oscillations in KS 1731-260
• Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?
• On the Correlated Spectral and Timing Properties of 4U 1636-53: an Atoll Source at High Accretion Rates
• On the high coherence of kilo-Hz Quasi-Periodic Oscillations
• The correlations between the twin kHz QPO frequencies of LMXBs
• Discovery of kHz QPOs and shifted frequency correlations in the accreting millisecond pulsar XTE J1807--294
• The Long Term Stability of Oscillations During Thermonuclear X-ray Bursts: Constraining the Binary X-ray Mass Function
• Does Turbulent Pressure Behave as a Logatrope?
• Supporting evidence for the signature of the innermost stable circular orbit in Rossi X-ray data from 4U1636-536
• Discovery of the Spin Frequency of 4U 0614+09 with SWIFT/BAT
• Discovery of Millisecond Variability in the Neutron-Star X-Ray Transient SAX J1750.8-2900
• Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification
• Timing properties and spectral states in Aquila X-1
• The correlations between the spin frequencies and kHz QPOs of Neutron Stars in LMXBs
• Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst

Cited by in corpus (25)

• Detecting gravitational wave emission from the known accreting neutron stars
• Accreting Millisecond X-Ray Pulsars
• The disc-jet coupling in the neutron star X-ray binary Aquila X-1
• On mass-constraints implied by the relativistic precession model of twin-peak quasi-periodic oscillations in Circinus X-1
• Higher order nonlinearity in accretion disks: QPOs of black hole and neutron star sources and their spin
• Measuring A Truncated Disk in Aquila X-1
• kHz Quasi-Periodic Oscillations in the low-mass X-ray binary 4U 0614+09
• Investigation of the emission radii of kHz QPOs for the accreting millisecond X-Ray pulsars, Atoll and Z sources
• Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821
• What can we extract from quasi-periodic oscillations?
• Statistical properties of twin kHz QPO in neutron star LMXBs
• Discovery of a 7 mHz X-Ray Quasi-periodic Oscillation from the most Massive Stellar-mass Black Hole IC 10 X-1
• Constraints on kHz QPO models and stellar EOSs from SAX J1808.4-3658, Cyg X-2 and 4U 1820-30
• Simple analytic formula relating the mass and spin of accreting compact objects to their rapid X-ray variability
• Correlations between the frequencies of twin kHz QPOs and spins of neutron stars in LMXBs
• Detection of a strong ~2.5 Hz modulation in the Newly Discovered Millisecond Pulsar MAXI J1816-195
• Spectral-Timing Analysis of the Lower kHz QPO in the Low-Mass X-ray Binary Aquila X-1
• Parallel tracks as quasi-steady states for the magnetic boundary layers in neutron-star low-mass X-ray binaries
• A model for upper kHz QPO coherence of accreting neutron star
• Probing the accretion disc structure by the twin kHz QPOs and spins of neutron stars in LMXBs
• Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity
• Statistical Analysis for the Q-factor of twin kHz QPOs
• Morphological analysis on the coherence of kHz QPOs
• QPO signatures of disk restoration after type-I X-ray bursts from 4U~1636$-$536
• Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT