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We analysed an XMM-Newton plus a simultaneous Rossi X-ray Timing Explorerobservation and a separate Suzaku observation of the neutron-star low-massX-ray binary 4U 1728-34. We fitted the X-ray spectra with the self-consistentreflection model relxill. We found that the inclination angle of 4U 1728-34 is49 degrees, consistent with the upper limit of 60 degrees deduced from theabsence of eclipses or dips in this source. The inclination angle in the fit ofthe XMM-Newton/RXTE observation is larger than 85 degrees, which may be due tothe possible calibration issues of the PN instrument in timing mode. We alsofound that the thermal emission from the accretion disc is not significant.This could be explained either by the relatively high column density of theinterstellar medium along the line of sight to the source, which decreases thenumber of soft disc photons, or if most of the soft thermal photons from thedisc are reprocessed in the corona. The ionisation parameter derived from thefits is larger than the value predicted in the framework of the standardreflection model, wherein the disc is irradiated by an X-ray source above thecompact object. This inconsistency suggests that irradiation from the neutronstar and its boundary layer may play an important role in the ionisation of theaccretion disc, and hence in the reflection component in this source.

Ionisation state of the accretion disc in the neutron-star low-mass X-ray binary 4U 1728-34