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Both the broad iron (Fe) line and the frequency of the kilohertzquasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries(LMXBs) can potentially provide independent measures of the inner radius of theaccretion disc. We use XMM-Newton and simultaneous Rossi X-ray Timing Explorerobservations of the LMXB 4U 1636-53 to test this hypothesis. We study theproperties of the Fe-K emission line as a function of the spectral state of thesource and the frequency of the kHz QPOs. We find that the inner radius of theaccretion disc deduced from the frequency of the upper kHz QPO varies as afunction of the position of the source in the colour-colour diagram, inaccordance with previous work and with the standard scenario of accretion discgeometry. On the contrary, the inner disc radius deduced from the profile ofthe Fe line is not correlated with the spectral state of the source. The valuesof the inner radius inferred from kHz QPOs and Fe lines, in four observations,do not lead to a consistent value of the neutron star mass, regardless of themodel used to fit the Fe line. Our results suggest that either the kHz QPO orthe standard relativistic Fe line interpretation does not apply for thissystem. Furthermore, the simultaneous detection of kHz QPOs and broad Fe linesis difficult to reconcile with models in which the broadening of the Fe line isdue to the reprocessing of photons in an outflowing wind.

Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636-53