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Ionisation state of the accretion disc in the neutron-star low-mass X-ray binary 4U 1728-34
We analysed an XMM-Newton plus a simultaneous Rossi X-ray Timing Explorer
observation and a separate Suzaku observation of the neutron-star low-mass
X-ray binary 4U 1728-34. We fitted the X-ray spectra with the self-consistent
reflection model relxill. We found that the inclination angle of 4U 1728-34 is
49 degrees, consistent with the upper limit of 60 degrees deduced from the
absence of eclipses or dips in this source. The inclination angle in the fit of
the XMM-Newton/RXTE observation is larger than 85 degrees, which may be due to
the possible calibration issues of the PN instrument in timing mode. We also
found that the thermal emission from the accretion disc is not significant.
This could be explained either by the relatively high column density of the
interstellar medium along the line of sight to the source, which decreases the
number of soft disc photons, or if most of the soft thermal photons from the
disc are reprocessed in the corona. The ionisation parameter derived from the
fits is larger than the value predicted in the framework of the standard
reflection model, wherein the disc is irradiated by an X-ray source above the
compact object. This inconsistency suggests that irradiation from the neutron
star and its boundary layer may play an important role in the ionisation of the
accretion disc, and hence in the reflection component in this source.Comment: 11 pages, 3 figures, accepted by MNRAS for publicatio
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