NuSTAR and SWIFT joint view of neutron star X-ray binary 4U 1728–34 : Disc reflection in the island and lower banana states

We analyze two simultaneous NuSTAR and SWIFT data of the Atoll type neutron star (NS) X-ray binary 4U 1728–34 observed on 1 and 3 October, 2013. Based on power density spectra, hardness ratio and colour-colour diagram, we infer that the first and second observations belong to the island state and lower banana state, respectively. During island state, a low-frequency quasi-periodic oscillation (LFQPO) at ∼ 4.53 Hz is observed along with four type-I X-ray bursts (∼ 10−15 s duration each). The X-ray luminosity of the source during the island and lower banana states are found to be LX = 1.1 and 1.6 × 10 37 erg s, respectively which correspond to ∼ 6% and ∼ 9% of the Eddington luminosity. The combined burst spectrum is well represented by a blackbody with a characteristic temperature of 2.22± 0.05 keV and the blackbody radius of 9.87±1.86 km which is consistent with the typical radius of the neutron star. The persistent energy spectra from both observations in the energy band 1− 79 keV are well described with thermal emission from the NS surface (kTbb ≃ 1 − 2.5 keV), Comptonized emission of thermal seed photons from the hot boundary layer/corona and the strong reflection component from the accretion disc. We detect a broad Iron line in the 5 − 8 keV band and reflection hump in the 15 − 30 keV band. These features are well modelled by the relxill reflection model. From joint spectral fitting, we constrain the inclination angle of the binary system and inner disc radius to be 22◦ − 40◦ and (2.8 − 4.3)×RISCO, respectively. From inner disc radius, we estimate the magnetic field to be (3.3− 6.5)× 10 Gauss.