NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus X-2

We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X-2 performed on 2015 January 7. During this observation, the source exhibited a sudden decrease in count rate (dips) and stronger variability in 3-79 keV X-ray light curve. The hardness-intensity diagram shows that the source remained in the so-called normal branch of the Z-track, although an extended `flaring branch' is observed during the dips. The source was in a soft spectral state with the 3-45 keV luminosity of L ≃ (0.5-1.1) × 10³⁸ erg s^-1, assuming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models in which the soft band is dominated by the emission from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe K α emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of ∼2. The relativistic reflection model predicts the inner radius of the accretion disc as R_in ≃ 2.5-6.0 R_ISCO (≃30-73 km) for the non-dip state and R_in ≃ 2.0-2.6 R_ISCO (≃24-32 km) for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this implies an upper limit of the magnetic field strength of ≤7.6 × 10⁹ G at the magnetic poles which is consistent with other estimates.