| Iron (Fe) K-alpha lines have now been detected in about half a dozen neutron-star accreting X-ray binaries. Under the commonly accepted interpretation, these lines can be used to set tight constrains on the accretion disc geometry and on the radius of the neutron star. In two of these systems, the inner radius of the accretion disc, as inferred from the line profile, appears to be consistent with the radius inferred from the frequency of the so-called kilohertz quasi-periodic oscillations (kHz QPOs). However, the observations used to measure the Fe K-alpha lines and kHz QPOs in those two sources were not strictly simultaneous. We have recently shown that contemporaneous XMM-Newton/RXTE and Chandra/RXTE observations of the neutron-star system 4U 1636--53 appear to contradict this picture: The inner disc radius inferred from the Fe line and the kHz QPO is not the same. Since the frequencies of the kHz QPOs are well correlated with the inner disc radius inferred from broad-band spectral fitting, it is intriguing (and somewhat worrisome) that the Fe line would behave differently. We have been recently awarded 115 kiloseconds with XMM-Newton (proposal category A) and simultaneous RXTE observations on the source 4U 1636--53. This source currently follows a $\sim 40$-day pattern of transitions between spectral states (presumably due to changes of the inner disc radius), therefore our program will sample a wide range of inner disc radii. |
