We have discovered a 7.06 \pm 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 5.4 \times 10 ^ { 37 } erg s ^ { -1 } ( for a distance of 6.4 kpc ) .
The QPO had a FWHM of only 0.5 \pm 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 % \pm 0.2 % .
The amplitude increased with photon energy from 3.7 % \pm 0.5 % between 2.8 and 5.3 keV to 7.3 % \pm 0.6 % between 6.8 and 9.3 keV , above which it remained approximately constant at \sim 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 \pm 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 \sim 20 % ( for a helium companion star ) of the Eddington accretion limit .