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

Abstract

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 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 \quotes{normal branch} of the Z-track, although an extended \quotes{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 $R_{in}\simeq2.5-6.0\:R_{ISCO}\:(\simeq30-73$ Km) for the non-dip state and $R_{in}\simeq2.0-2.6\:R_{ISCO}\:(\simeq24-32$ 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 $\leq7.6\times10^{9}$ G at the magnetic poles which is consistent with other estimates.