Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

TL;DR

This study analyzes the polarization of Cygnus X-3 in its hard state, revealing remarkable stability over a year, which informs models of accretion geometry and emission mechanisms in ultraluminous X-ray sources.

Contribution

It provides the first detailed polarization measurements of Cygnus X-3 in the hard state, demonstrating long-term stability and insights into accretion geometry.

Findings

Polarization degree of 21.2% in 2023 consistent with 2022 observations

Polarization angle of 92.2° indicates stable accretion geometry

Reflection from an optically thick envelope dominates emission

Abstract

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.