Strong-field gravity and X-ray observations of 4U 1820-30

The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kH z in the X-ray emission from the neutron star X-ray binary 4U 1820-30 has b een interpreted as evidence for the existence of the marginally stable orbi t, a key prediction of strong-field general relativity. The signature of th e marginally stable orbit is a saturation in QPO frequency (assumed to trac k inner disk radius) versus mass accretion rate. Previous studies of 4U 182 0-30 have used X-ray count rate as an indicator of mass accretion rate. How ever, X-ray count rate is known to not correlate robustly with mass accreti on rate or QPO frequency in other sources. Here, we examine the QPO frequen cy dependence on two other indicators of mass accretion rate: energy flux a nd 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.