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 as a function of the position of the source in the colour-colour diagram. We modelled the spectra with two thermal components, a multicolour disc blackbody and a blackbody, that represent, respectively, the emission from the accretion disc and the neutron-star surface plus boundary layer, a Comptonised component to account for the hard part of the spectrum produced by a corona of hot electrons, and either a Gaussian or a relativistic line component to model an iron emission line at 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, in which the inner disc temperature, the temperature at the surface of the neutron star (or boundary layer) and the optical depth of the corona increase while the inner disc radius and the temperature of the corona decrease as mass accretion rate onto the neutron star increases. We also find that the flux and equivalent width of the iron line first increases and then decreases as the flux of the Comptonised component increases. This could be explained either by changes in the ionisation state of the accretion disc where the line is produced by reflection, or by light bending of the emission from the Comptonised component if the height at which this component is produced changes with mass accretion rate.