Discovery of a ∼ 7 Hz Quasi - Periodic Oscillation in the Low - Luminosity Low - Mass X - Ray Binary 4 U 1820 – 30

We have discovered a 7.06±0.08 Hz quasi-periodic oscillation (QPO) in the X-ray flux of the lowluminosity 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 5.4×10 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 nearEddington 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. Subject headings: accretion, accretion disks — stars: individual (4U 1820–30; NGC 6624) — stars: neutron — X-rays: stars