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

Abstract

<p> 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 <em>IXPE, NICER</em>, and <em>INTEGRAL</em>. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with <em>kT</em>_in ≈ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature <em>kT</em>_e = 3 keV and optical depth τ ≈ 4, assuming a slab geometry. We measure the polarization degree in the 2–8 keV band <em>P</em> = 1.8 ± 0.3 per cent and polarization angle ϕ = 140° ± 4°, consistent with the previous X-ray polarimetric measurements by <em>OSO-8</em> 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 <em>IXPE</em> data, the Comptonized emission has a polarization degree <em>P</em> = 4.0 ± 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. <br></p>