Probing the gravitational field around neutron stars with Fe lines and kHz QPOs

Broad Fe lines have been detected in ten LMXBs. In two of these sources the inner accretion-disc radii inferred from the line and from the kHz QPOs are consistent, but the line and QPO measurements were not simultaneous. Using contemporaneous XMM-Newton/RXTE and Chandra/RXTE observations we have recently shown that in the LMXB 4U 1636-53 the Fe line and the kHz QPO yield different disc radii. The frequencies of the kHz QPOs correlate with the disc radius inferred from broad-band spectra, with the Fe line changing independently. We propose five simultaneous XMM-Newton/RXTE observations of the LMXB 4U 1728-34 with both a broad Fe line and kHz QPOs, using the long-term variability of this source to test the dynamics and the correlation of the radii inferred from the line and the kHz QPOs.