  The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz
in the x-ray emission from the neutron star x-ray binary 4U1820-30 has been
interpreted as evidence for the existence of the marginally stable orbit, a key
prediction of strong-field general relativity. The signature of the marginally
stable orbit is a saturation in QPO frequency, assumed to track inner disk
radius, versus mass accretion rate. Previous studies of 4U1820-30 have used
x-ray count rate as an indicator of mass accretion rate. However, x-ray count
rate is known to not correlate robustly with mass accretion rate or QPO
frequency in other sources. Here, we examine the QPO frequency dependence on
two other indicators of mass accretion rate: energy flux and x-ray spectral
shape. Using either of these indicators, we find that the QPO frequency
saturates at high mass accretion rates. We interpret this as strong evidence
for the existence of the marginally stable orbit.
