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

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 $11.9\pm 0.5\%$ at a polarization angle (PA) of $94^{\circ }\pm 1^{\circ }$ 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.

9 pages, 11 figures, submitted to A&A

References in corpus (10)

• Analyzing the Data from X-ray Polarimeters with Stokes Parameters
• Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission
• Handling Background in IXPE polarimetric data
• A weighted analysis to improve the X-ray polarization sensitivity of IXPE
• Relativistic rotating vector model for X-ray millisecond pulsars
• The nature of the hard state of Cygnus X-3
• Spectral variability in Cygnus X-3
• Gemini/GNIRS infrared spectroscopy of the Wolf-Rayet stellar wind in Cygnus X-3
• Near-IR and X-ray Variability of Cyg X-3: Evidence for a Compact IR Source and Complex Wind Structures
• Photoionization Emission Models for the Cyg X-3 X-ray Spectrum

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• First Detection of X-ray Polarization and Its Short-term Increase Pre- and Post-Eclipse in HMXB 4U 1700-377
• Relativistic Scattering in the Funnel of Cygnus X-3