Kilohertz quasi-periodic oscillations and broad iron emission lines as a probe of strong-field gravity.

Kilohertz quasi-periodic oscillations (kHz QPOs) and broad iron emission lines are thought to be produced at the inner edge of an accretion disk around neutron stars in low mass X-ray binary systems (LMXBs). The frequency of kHz QPOs and the width of iron lines strongly suggest that these phenomena take place just few kilometers above the surface of the neutron star and hence they should probe the strong gravitational field around these stars. Both QPO variability and iron line hold the promise of searching for signatures of strong-field general relativity, e.g. the ISCO or Lense-Thirring precession. Here I present new results of the kHz QPOs in the transient neutron star LMXB XTE J1701-462, and the kHz QPOs and the broad iron line in the persistent neutron star LMXB 4U 1636-53. In XTE J1701 we find that, contrary to previous claims, properties of the kHz QPO are probably not uniquely determined by the geometry of the space-time. In 4U 1636 we find that there is a complex relation between kHz QPOs and properties of the iron line, which is difficult to explain if both phenomena reflect properties of the same region in the accretion disk.