2023MNRAS.521.4792J

We present a detailed spectral-timing analysis of the normal branch oscillations (NBOs) observed in two Z sources, Cyg X-2 and Sco X-1, using the data of the Neutron star Interior Composition Explorer (NICER). This is the first study of the NBO properties down to ∼0.4 keV. For Cyg X-2, we confirm the presence of a pivot energy at around 5 keV in the root mean square (rms) spectrum and the phase jump at the pivot energy. In addition, we find a positive phase lag of lower energy (down to 0.4 keV) photons with respect to photons in 2-2.5 keV. For Sco X-1, a rms pivot energy at ∼1.5 keV is detected for the first time, and at the pivot energy a large phase shift is observed. These results indicate that the NBO signal is associated with two different spectral components. By fitting the energy spectra of the states with and without the NBO signal using a two-COMPTB model, we find that the emission of the NBO below or above ∼1.5 keV is mainly related to the Comptonization of the transition layer with the photons from the accretion disc or the neutron star, respectively. In addition, we suggest that if the accretion rate in a Z source is high enough to change the optical depth of the transition layer significantly, a phase jump will occur. A phase shift will be present due to the mixing of the two Comptonization components.