Discovery of a ~7 Hz Quasi-Periodic Oscillation in the low-luminosity low-mass X-ray binary 4U 1820-30

Abstract

We have discovered a 7.06+-0.08 Hz quasi-periodic oscillation (QPO) in the X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source 4U 1820-30. This QPO was only observable at the highest observed mass accretion rate, when the source was in the uppermost part of the banana branch, at a 2-25 keV luminosity of 5.4x10^37 erg/s (for a distance of 6.4 kpc). The QPO had a FWHM of only 0.5+-0.2 Hz during small time intervals (32-s of data), and showed erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms amplitude over the energy range 2-60 keV was 5.6%+-0.2%. The amplitude increased with photon energy from 3.7%+-0.5% between 2.8 and 5.3 keV to 7.3%+-0.6% between 6.8 and 9.3 keV, above which it remained approximately constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was consistent with being zero (-1.2+-3.4 ms). The properties of the QPO (i.e., its frequency and its presence only at the highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO observed in the highest luminosity LMXBs (the Z sources) when they are accreting near the Eddington mass accretion limit. If this is indeed the same phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which require the near-Eddington accretion rates, will not hold. Assuming isotropic emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is at maximum only 40% (for a companion star with cosmic abundances), but most likely ~20% (for a helium companion star) of the Eddington accretion limit.

Accepted for publication in ApJ Letters (6 pages, including 3 figures)

References in corpus (6)

• Correlation Between Energy Spectral States and Fast Time Variability and Further Evidence for the Marginally Stable Orbit in 4U 1820-30
• Strong Correlation Between Noise Features at Low Frequency and the Kilohertz QPOs in the X-Ray Binary 4U 1728-34
• The broad-band power spectrum of SAX J1808.4-3658
• Time-Resolved Ultraviolet Observations of the Globular Cluster X-ray Source in NGC 6624: The Shortest Known Period Binary System
• Discovery of KiloHertz Quasi-Periodic Oscillations in 4U 1735-44
• Discovery of Kilohertz QPOs in the Atoll X-Ray Binary 4U 1705-44

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• The Complex Phase Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550-564
• RXTE observations of the neutron star low-mass X-ray binary GX 17+2: correlated X-ray spectral and timing behavior
• Binary and Long-Term (Triple?) Modulations of 4U 1820$-$30 in NGC 6624
• Study of the Temporal Behavior of 4U 1728-34 as a Function of its Position in the Color-Color Diagram
• The power spectral properties of the Z-source GX 340+0
• Bardeen-Petterson Effect and Quasi-Periodic Oscillations in X-Ray Binaries
• On the Correlated Spectral and Timing Properties of 4U 1636-53: an Atoll Source at High Accretion Rates
• KiloHertz quasi-periodic oscillations difference frequency exceeds inferred spin frequency in 4U 1636-53
• Timing properties and spectral states in Aquila X-1
• Peculiar spectral and power spectral behaviour of the LMXB GX 13+1
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• Dependence of kHz QPO Properties on Normal-Branch Oscillation Phase in Scorpius X-1
• Spectral state evolution of 4U~1820-30: the stability of the spectral index of Comptonization tail
• A continuous Flaring- to Normal-branch transition in Sco X-1
• Quasi-periodic oscillations and noise in neutron star and black-hole X-ray binaries
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• Eddington Capture Sphere around luminous stars
• Normal-Branch Quasi-Periodic Oscillations during the High Intensity State of Cygnus X-2
• Coupled HBO and NBO variations in the Z source GX 5-1: inner accretion disk as the location of QPOs
• An XTE Archival Search for Coherent X-ray Pulsations in LMXB 4U 1820-30
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