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

We study 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 (BAT) 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 phenomenon of black hole transients. In SIMS, power density spectra were dominated by broadband noise components, and two type B QPOs 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 were studied. On MJD 58584.69, a small-scale reflare happened, and we observed that the spectral index decreased to a much lower value associated with finite changes in other spectral parameters also, 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, which corresponds to the Innermost Stable Circular Orbit (ISCO). We have measured the lower limit of mass of the compact object to be approximately 4.6 M, considering a non-spinning black hole binary system, by fitting 1-10 keV NICER spectra with the diskbb component. 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.