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≃(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 Kα 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 Rin≃2.5−6.0RISCO(≃30−73 Km) for the non-dip state and Rin≃2.0−2.6RISCO(≃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.
@article{arxiv.1708.01032,
title = {NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus~X--2},
author = {Aditya S. Mondal and Gulab C. Dewangan and Mayukh Pahari and Biplab Raychaudhuri},
journal= {arXiv preprint arXiv:1708.01032},
year = {2017}
}