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

Broad Fe lines have been detected in 10 neutron-star LMXBs. In two sources the Fe line and kHz QPOs give consistent values for the inner radius of the accretion disc, but measurements were not simultaneous. Using XMM-Newton/RXTE and Chandra/RXTE simultaneously, 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, while the Fe line appears to change independently. We propose 5 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 correlation of the radii inferred from the line and the kHz QPOs.