Properties of kilohertz quasi-periodic oscillations, and the signature of the marginally stable orbit around neutron stars

Kilohertz quasi-periodic oscillations (kHz QPOs) offer the exciting possibility of detecting, for the first time, the presence of the marginally stable orbit (MSO) around a neutron star. Since the MSO has an impact on the dynamics of matter in orbit around the neutron star, if the kHz QPOs are produced in an accretion disc around the neutron star the MSO should also affect the properties of these QPOs. Models predict that all three characteristic properties of the kHz QPOs, their central frequency, amplitude, and coherence, should display specific trends as the radial position in the accretion disc where the oscillations are produced approaches the radius of the MSO. Interestingly, these trends have been observed in a handful of sources of kHz QPOs. I have recently argued that (part of) those trends may in fact be due to changes in the properties of the accreting gas that modulates the X-ray flux that makes the QPOs visible. Here I will summarize the status of this conundrum, and I will show results from recent observations of a source of kHz QPOs that appear to resolve this issue