Study of Type-B QPOs Observed in Black Hole X-Ray Binary Swift J1728.9-3613

We report the detection of type-B quasi-periodic oscillation (QPO) of the black hole X-ray binary Swift J1728.9-3613 observed by NICER during the 2019 outburst. A type-B QPO was observed for the first two days and it disappeared as flux increased, but again appeared at ∼7.70 Hz when flux was dramatically decreased. The source was found in the soft intermediate state during these observations. We further studied the energy dependence of the QPO. We found that QPO was observed only for a higher energy range implying that the origin of QPO is possibly due to the corona emitting higher energy photons by the inverse Compton process. The variation of spectral parameters can be explained with the disk truncation model. The fractional rms was found to be monotonically increased with energy. The phase lag spectrum followed the "U-shaped" curve. The rms and phase lag spectrum are modeled and explained with the single-component Comptonization model vkompthdk.