X-Ray Polarized View on the Accretion Geometry in the X-Ray Binary Circinus X-1

Abstract

Cir X-1 is a neutron star X-ray binary characterized by strong variations in flux during its eccentric $\sim$16.6 days orbit. There are also strong variations in the spectral state, and historically it has shown both atoll and Z state properties. We observed the source with the Imaging X-ray Polarimetry Explorer during two orbital segments, 6 days apart, for a total of 263~ks. We find an X-ray polarization degree in these segments of $1.6\%\pm 0.3\%$ and $1.4\%\pm 0.3\%$ at polarization angles of $37^{\circ }\pm 5^{\circ }$ and $-12^{\circ }\pm 7^{\circ }$, respectively. Thus we observed a rotation of the polarization angle by $49^{\circ }\pm 8^{\circ }$ along the orbit. Because variations of accretion flow, and then of the hardness ratio, are expected during the orbit, we also studied the polarization binned in hardness ratio, and found the polarization angle differing by $67^{\circ }\pm 11^{\circ }$ between the lowest and highest values of the hardness ratio. We discuss possible interpretations of this result that could indicate a possible misalignment between the symmetry axes of the accretion disk and the Comptonizing region caused by the misalignment of the neutron star's angular momentum with respect to the orbital one.

arXiv comment15 pages, 6 figures, Accepted for publication in ApJL

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• New polarimetric study of the galactic X-ray burster GX 13+1
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• X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?
• Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry
• X-ray Dips and Polarization Angle Swings in GX 13+1
• X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch
• IXPE view of the Sco-like source GX 349+2 in the normal branch