2018MNRAS.474.2064M -
Mon. Not. R. Astron. Soc., 474, 2064-2072 (2018/February-3)
NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus X-2.
MONDAL A.S., DEWANGAN G.C., PAHARI M. and RAYCHAUDHURI B.
Abstract (from CDS):
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) x 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 Rin ≃ 2.5-6.0 RISCO (≃30-73 km) for the non-dip state and Rin ≃ 2.0-2.6 RISCO (≃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 x 109 G at the magnetic poles which is consistent with other estimates.
Abstract Copyright:
© 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
Journal keyword(s):
accretion, accretion discs - stars: individual: Cyg X-2 - stars: neutron - X-rays: binaries
Simbad objects:
6
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