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Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

...+91
Dec 2022

We report spectro-polarimetric results of an observational campaign of thebright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed byIXPE, NICER and INTEGRAL. Consistently with previous results, the broad-bandspectrum is characterized by a lower-energy component, attributed to theaccretion disc with $kT_{\rm in} \approx$ 1 keV, plus unsaturatedComptonization in thermal plasma with temperature $kT_{\rm e} = 3$ keV andoptical depth $\tau \approx 4$, assuming a slab geometry. We measure thepolarization degree in the 2-8 keV band $P=1.8 \pm 0.3$ per cent andpolarization angle $\phi = 140^{\circ} \pm 4^{\circ}$, consistent with theprevious X-ray polarimetric measurements by OSO-8 as well as with the directionof the radio jet which was earlier observed from the source. While polarizationof the disc spectral component is poorly constrained with the IXPE data, theComptonized emission has a polarization degree $P =4.0 \pm 0.7$ per cent and apolarization angle aligned with the radio jet. Our results strongly favour aspreading layer at the neutron star surface as the main source of thepolarization signal. However, we cannot exclude a significant contribution fromreflection off the accretion disc, as indicated by the presence of the ironfluorescence line.

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