Discovery of Two Simultaneous Kilohertz Quasi-Periodic Oscillations in KS 1731-260

Abstract

We have discovered two simultaneous quasi-periodic oscillations (QPOs) at 898.3+/-3.3 Hz and 1158.6+/-9.0 Hz in the 1996 August 1 observation of the low-mass X-ray binary KS 1731-260 with the Rossi X-ray Timing Explorer. The rms amplitude and FWHM of the lower frequency QPO were 5.3+/-0.7 % and 22+/-8 Hz, whereas those of the higher frequency QPO were 5.2+/-1.0 % and 37+/-21 Hz. At low inferred mass accretion rate both QPOs are visible, at slightly higher mass accretion rate the lower frequency QPO disappears and the frequency of the higher frequency QPO increases to ~1178 Hz. At the highest inferred mass accretion rate this QPO is only marginally detectable (2.1 sigma) near 1207 Hz, which is the highest frequency so far observed in an X-ray binary. The frequency difference (260.3+/-9.6 Hz) between the QPOs is equal to half the frequency of the oscillations observed in a type I burst in this source (at 523.92+/-0.05 Hz, Smith, Morgan and Bradt 1997). This suggests that the neutron star spin frequency is 261.96 Hz (3.8 ms), and that the lower frequency QPO is the beat between the higher frequency QPO, which could be a preferred orbital frequency around the neutron star, and the neutron star spin. During the 1996 August 31 observation we detected an additional QPO at 26.9+/-2.3 Hz, with a FWHM and rms amplitude of 11+/-5 Hz and 3.4+/-0.6 %.

6 pages including 3 figures, Astrophysical Journal Letters, in press (issue 482)

References in corpus (2)

• Kilohertz QPO Peak Separation Is Not Constant in Scorpius X-1
• RXTE Discovery of Coherent Millisecond Pulsations during an X-ray Burst from KS 1731-260

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• Detecting gravitational wave emission from the known accreting neutron stars
• Hard X-ray Emission From Low Mass X-ray Binaries
• Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52
• Bounds on the Compactness of Neutron Stars from Brightness Oscillations
• On the Amplitude of Burst Oscillations in 4U 1636-54: Evidence for Nuclear Powered Pulsars
• Relations Between Timing Features and Colors in the X-Ray Binary 4U 0614+09
• The distribution of kHz QPO frequencies in bright LMXBs
• Kilohertz Quasi-Periodic Oscillation Peak Separation is not Constant in the Atoll Source 4U 1608-52
• Cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29
• Discovery of a second kHz QPO peak in 4U 1608-52
• Nearly Coherent Oscillations in Type I X-Ray Bursts from KS 1731-260
• Relativistic precession around rotating neutron stars: Effects due to frame-dragging and stellar oblateness
• The Comptonising medium of the neutron star in 4U 1636-53 through its lower kilohertz Quasi-Periodic Oscillations
• Modelling the spin equilibrium of neutron stars in LMXBs without gravitational radiation
• KiloHertz Quasi-Periodic Oscillations in GX 17+2
• Changing Frequency Separation of Kilohertz Quasi-Periodic Oscillations in the Sonic-Point Beat-Frequency Model
• A half-a-day long thermonuclear X-ray burst from KS 1731-260
• On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries
• The Beat-Frequency Interpretation of Kilohertz QPOs in Neutron Star Low-Mass X-ray Binaries
• Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?
• On the Frequency Evolution of X-ray Brightness Oscillations During Thermonuclear X-ray Bursts: Evidence for Coherent Oscillations
• Discovery of kHz QPOs and shifted frequency correlations in the accreting millisecond pulsar XTE J1807--294
• Effects of Rapid Stellar Rotation on Equation of State Constraints Derived from Quasi-Periodic Brightness Oscillations
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• Difference Frequency of Kilohertz QPOs Not Equal to Half the Burst Oscillation Frequency in 4U 1636-53
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• Discovery of a Second Kilohertz QPO in the X-ray Binary 4U 1735-44
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• Oscillations During Thermonuclear X-ray Bursts
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• Discovery of the upper kilo-Hz QPO from the X-ray transient Aql X-1
• Disk Accretion onto Magnetized Neutron Stars: The Inner Disk Radius and Fastness Parameter
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• ASCA Observations of GX 354-0 and KS 1731-260
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• Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
• Rotational parameters of strange stars in comparison with neutron stars
• The Infrared Counterpart of the X-ray Burster KS 1731--260
• Correlations between the frequencies of twin kHz QPOs and spins of neutron stars in LMXBs
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• Thermonuclear Burning on Rapidly Accreting Neutron Stars
• Testing the transition layer model of quasi-periodic oscillations in neutron star X-ray binarie
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