Study of the X-ray properties of the neutron-star binary 4U 1728$-$34 from the soft to hard state

Abstract

We studied five XMM-Newton observations of the neutron-star binary 4U 1728$-$34 covering the hard, intermediate and soft spectral states. By jointly fitting the spectra with several reflection models, we obtained an inclination angle of 25$-$53$°$ and an iron abundance up to 10 times the solar. From the fits with reflection models, we found that the fluxes of the reflection and the Comptonised components vary inconsistently; since the latter is assumed to be the illuminating source, this result possibly indicates the contribution of the neutron star surface/boundary layer to the disc reflection. As the source evolved from the relatively soft to the intermediate state, the disc inner radius decreased, opposite to the prediction of the standard accretion disc model. We also explore the possible reasons why the supersolar iron abundance is required by the data and found that this high value is probably caused by the absence of the hard photons in the XMM-Newton data.

arXiv comment14 pages, 6 figures

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• UltraCompCAT: a comprehensive Catalogue of Ultra-Compact and Short Orbital Period X-ray Binaries
• A variable ionized disk wind in the black-hole candidate EXO 1846-031
• Spectro-polarimetric view of bright atoll source GX 9+9 using IXPE and AstroSat
• Thermonuclear explosions on neutron stars reveal the speed of their jets
• Discovery of a thermonuclear Type I X-ray burst in infrared: new limits on the orbital period of 4U 1728-34
• High-frequency variability in neutron-star low-mass X-ray binaries
• A retrograde spin of the black hole in MAXI J1659--152
• AstroSat view of `Clocked` burster GS 1826-238:broad-band spectral nature of persistent and burst emission during soft state
• Probing the accretion geometry of the atoll source 4U 1702-429 in different spectral states with NICER, NuSTAR, and AstroSat
• Correlated spectro-polarimetric study along the Z track in XTE J1701-462 puts constraints on its coronal geometry
• Modeling the upper kHz QPOs of 4U 1728-34 with X-ray reverberation