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We analyze two simultaneous \nustar{} and \swift{} data of the Atoll typeneutron 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, weinfer that the first and second observations belong to the island state andlower banana state, respectively. During island state, a low-frequencyquasi-periodic oscillation (LFQPO) at $\sim4.53$ Hz is observed along with fourtype-I X-ray bursts ($\sim 10-15$ s duration each). The X-ray luminosity of thesource during the island and lower banana states are found to be $L_{X}= 1.1$and $1.6\times10^{37}$ erg s$^{-1}$, respectively which correspond to $\sim6\%$ and $\sim 9\%$ of the Eddington luminosity. The combined burst spectrum iswell represented by a blackbody with a characteristic temperature of$2.22\pm0.05\kev$ and the blackbody radius of $9.87\pm1.86$ km which isconsistent with the typical radius of the neutron star. The persistent energyspectra from both observations in the energy band $1-79\kev$ are well describedwith thermal emission from the NS surface ($kT_{bb}\simeq1-2.5\kev$),Comptonized emission of thermal seed photons from the hot boundary layer/coronaand the strong reflection component from the accretion disc. We detect a broadIron line in the $5-8 \kev$ band and reflection hump in the $15-30 \kev$ band.These features are well modelled by the {\tt relxill} reflection model. Fromjoint spectral fitting, we constrain the inclination angle of the binary systemand inner disc radius to be $22\degree-40\degree$ and $(2.8-4.3)\timesR_{ISCO}$, respectively. From inner disc radius, we estimate the magnetic fieldto be $(3.3-6.5)\times10^{8}$ Gauss.

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