Ultrasoft state of microquasar Cygnus X-3: X-ray polarimetry reveals the geometry of astronomical puzzle

TL;DR

This study uses X-ray polarimetry to analyze the ultrasoft state of Cygnus X-3, revealing the geometry of the system and suggesting that the central source is obscured by an optically thick outflow, explaining its state changes.

Contribution

The paper provides the first polarimetric analysis of Cygnus X-3 in the ultrasoft state, revealing the system's geometry and the role of outflows in its state transitions.

Findings

Polarization degree of 11.9% indicates scattering by outflows.

Central source is hidden behind optically thick outflow.

Source inclination estimated between 26° and 28°.

Abstract

Cygnus X-3 is an enigmatic X-ray binary, that is both an exceptional accreting system and a cornerstone for the population synthesis studies. Prominent X-ray and radio properties follow a well-defined pattern, yet the physical reasons for the state changes observed in this system are not known. Recently, the presence of an optically thick envelope around the central source in the hard state was revealed using the X-ray polarization data obtained with Imaging X-ray Polarimetry Explorer (IXPE). In this work, we analyse IXPE data obtained in the ultrasoft (radio quenched) state of the source. The average polarization degree (PD) of $11.9\pm0.5\%$ at a polarization angle (PA) of $94^{\circ}\pm1^{\circ}$ is inconsistent with the simple geometry of the accretion disc viewed at an intermediate inclination. The high PD, the blackbody-like spectrum, and the weakness of fluorescent iron line imply that the central source is hidden behind the optically thick outflow and its beamed radiation is scattered towards our line of sight. In this picture the observed PD is directly related to the source inclination, which we conservatively determine to lie in the range $26^{\circ}<i<28^{\circ}$. Using the new polarimetric properties, we propose the scenario that can be responsible for the cyclic behaviour of the state changes in the binary.