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

TL;DR

This study uses X-ray polarization measurements from IXPE, NICER, and INTEGRAL to analyze the accretion geometry of Cyg X-2, revealing polarization aligned with the radio jet and supporting a spreading layer at the neutron star surface.

Contribution

First simultaneous spectro-polarimetric observations of Cyg X-2 across multiple instruments, providing new insights into accretion geometry and polarization origins.

Findings

Polarization degree in 2-8 keV band is 1.8% with a 4° angle.

Comptonized emission shows 4.0% polarization aligned with the jet.

Results favor a spreading layer as the main polarization source.

Abstract

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