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We report the detection of type-B quasi-periodic oscillation (QPO) of theblack hole X-ray binary Swift J1728.9-3613 observed by NICER during the 2019outburst. A type- B QPO was observed for the first two days and it disappearedas flux increased, but again appeared at $\sim$ 7.70 Hz when flux wasdramatically decreased. The source was found in the soft-intermediate stateduring 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 theorigin of QPO is possibly due to the corona emitting higher energy photons bythe inverse Compton process. The variation of spectral parameters can beexplained with the disk truncation model. The fractional rms found to bemonotonically increased with energy. The phase lag spectrum followed theU-shaped curve. The rms and phase lag spectrum are modelled and explained withthe single-component comptonization model vkompthdk.

Study of type-B QPOs observed in black hole X-ray binary Swift J1728.9-3613