Top: Temporal evolution of the RMS integrated over all frequencies (top panel) and QPO central frequency (medium panel), estimated from the 0.3–12 keV NICERPDS. The bottom panel presents the temporal evolution of the hardness ratio, as defined in Sect. 3.2. Middle: Two 0.3–12 keV NICERPDS showing the QPO for the highest (left, OBSID 6203690104) and lowest (right, OBSID 6203690114) central frequency ν_QPO. Poisson noise was not subtracted. Bottom: The PDS of the NuSTAR (black) and NICER (red) simultaneous observations in the common energy range of 3–10 keV. Poisson noise was subtracted for plotting purpose. The solid lines represent the best-fit models.