A Spectro-temporal View of Normal Branch Oscillations in Cygnus X-2 as Seen by NICER and NuSTAR

Abstract We report the spectro-temporal study of the neutron star low mass X-ray binary Cygnus X-2 using NICER and NuSTAR data while the source was in the normal branch (NB). We detect a normal branch oscillation (NBO) feature at ∼5.41 Hz that appears in the middle portion of the NB branch. We note that the NBO appeared only in the 0.5–3 keV energy range, with maximum strength in the 1–2 keV energy band, but was absent in the 3–10 keV energy band of NuSTAR and NICER data. The energy spectrum of the source exhibits an emission feature at ∼1 keV, previously identified as the Fe L transition in the outer region of the accretion disk. Upon considering both the Fe L and NBO features, we suggest that the originating location of the Fe L line and the NBOs may coincide and perhaps be due to the same underlying mechanism. Therefore, lags seen in the frequency-/energy-dependent lag spectra of Cygnus X-2 could be considered to be arising from a region of photoionized material far from the central source. We study the frequency and energy-dependent lag spectra of the source, which exhibited a few millisecond hard lag at the NBO frequency (12–15 ms) and a switch from hard to soft lags at 1 keV. The rms spectrum peaks at 1 keV, and the covariance spectrum clearly resembles a thermal spectrum. We discuss the spectro-temporal behavior of the NBO and attempt to constrain its location of origin.