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

The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz in the X-ray emission from the neutron star X-ray binary 4U 1820-30 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 4U 1820-30 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.