Swift J1728.9-3613 is a black hole X-ray binary: a spectral and timing study using NICER

We have studied different timing and spectral properties of the new Galactic X-ray transient Swift J1728.9-3613 using NICER and Swift, discovered by the Burst Alert Telescope on the Neil Gehrels Swift Observatory. The source went through multiple transitions to different spectral states during the outburst, and the complete evolution created a 'q'-shaped track in the hardness-intensity diagram. A partial hysteresis is also observed in the RMS-intensity diagram, which is another well-defined feature of black hole transients. In soft intermediate states, power-density spectra were dominated by broad-band noise components, and two type-B quasi-periodic oscillations were detected. We have fitted 1-10 keV energy spectra obtained from NICER observations that were performed during the outburst, and the temporal evolution of spectral parameters is studied. A small-scale reflare happened near MJD 58584.69, resulting in finite changes in spectral parameters, and the 1-10 keV averaged flux also increased. We observed that the innermost radius of the accretion disc was almost constant during the soft state and we have measured the mass of the compact object to be ~4.6 M_⊙, considering a non-spinning black hole binary system. The soft-to-hard transition occurred when the bolometric luminosity was 0.01 times the Eddington luminosity. Based on our combined study of the evolution of the timing and spectral properties, we conclude that the new source Swift J1728.9-3613 is a black hole X-ray binary.