Discovery of an ~7 HZ Quasi-periodic Oscillation in the Low-Luminosity Low-Mass X-Ray Binary 4U 1820-30

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 5.4×10³⁷ ergs s^-1 (for a distance of 6.4 kpc). The QPO had an 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 at only 40% (for a companion star with cosmic abundances), but most likely at ~20% (for a helium companion star), of the Eddington accretion limit.