Iron-line and continuum variations in the XMM-Newton and Suzaku spectra of the neutron-star low-mass X-ray binary 4U 1636-53

We used six simultaneous XMM-Newton and Rossi X-ray Timing Explorer plus five Suzaku observations to study the continuum spectrum and the iron emission line in the neutron-star low-mass X-ray binary 4U 1636-53. We modelled the spectra with two thermal components (representing the accretion disc and boundary layer), a Comptonized component (representing a hot corona), and either a Gaussian or a relativistic line component to model an iron emission line at similar to 6.5 keV. For the relativistic line component, we used either the DISKLINE, LAOR or KYRLINE model, the latter for three different values of the spin parameter. The fitting results for the continuum are consistent with the standard truncated disc scenario. We also find that the flux and equivalent width of the iron line first increase and then decrease as the flux of the Comptonized component increases. This could be explained either by changes in the ionization state of the accretion disc where the line is produced by reflection, or by light bending of the emission from the Comptonized component if the height at which this component is produced changes with mass accretion rate.