We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X–2 performed on 7 January 2015 .
During this observation , the source exhibited a sudden decrease in count rate ( dips ) and stronger variability in 3 - 79 { ~ { } keV } X-ray lightcurve .
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 \simeq ( 0.5 - 1.1 ) \times 10 ^ { 38 } 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 \alpha emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of \sim 2 .
The relativistic reflection model predicts the inner radius of the accretion disc as R _ { in } \simeq 2.5 - 6.0 \ > R _ { ISCO } \ > ( \simeq 30 - 73 Km ) for the non-dip state and R _ { in } \simeq 2.0 - 2.6 \ > R _ { ISCO } \ > ( \simeq 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 \leq 7.6 \times 10 ^ { 9 } G at the magnetic poles which is consistent with other estimates .