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

We analyse two simultaneous NuSTAR and Swift data of the Atoll-type neutron star (NS) X-ray binary 4U 1728-34 observed on 2013 October 1 and 3. We infer that the first and the second observations belong to the island state and the lower banana state, respectively. During island state, four type-I X-ray bursts are observed within 60 ks exposure. From the time-resolved spectral analysis of each burst with NuSTAR, the blackbody temperature kT_bb are found to vary between 1.3 and 3.0 keV, while the blackbody normalizations (km/10 kpc)² vary in the range 20-200, which translates to blackbody radii of 3.5-7.4 km for an assumed distance of 5 kpc. The persistent, joint energy spectra from Swift and NuSTAR for both observations in the energy band 1-79 keV are well described with thermal emission from the NS surface (kT_bb ≃ 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 modelled by the relxill reflection model. Joint spectral fitting constrains the inclination angle of the binary system and inner disc radius to be 22°-40° and (2.0-4.3) × R_ISCO, respectively. We estimate the magnetic field to be (1.8-6.5) × 10⁸ G. The X-ray luminosity of the source during the island and lower banana states are found to be L_X = 1.1 and 1.6 × 10³⁷ erg s^-1, respectively, which correspond to ∼6 per cent and ∼9 per cent of the Eddington luminosity.