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.

arXiv comment9 pages, 11 figures, submitted to A&A

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Cited by in corpus (11)

• X-ray polarization from accretion disk winds
• Particle acceleration and multi-messenger radiation from Ultra-Luminous X-ray Sources: A new class of Galactic PeVatrons
• Galactic transient sources with the Cherenkov Telescope Array Observatory
• Prospects for Time-Domain and Multi-Messenger Science with eXTP
• Analysis and simulations of binary black hole merger spins -- the question of spin-axis tossing at black hole formation
• Spectral Analysis of Cyg X-3 using simultaneous AstroSat and Insight-HXMT Observations
• First Detection of X-ray Polarization and Its Short-term Increase Pre- and Post-Eclipse in HMXB 4U 1700-377
• Shape and ionization of equatorial matter near compact objects from X-ray polarization reflection signatures
• The constraining power of X-ray polarimetry: detailed structure of the intrabinary bow shock in Cygnus X-3
• Birthplaces of X-ray emission lines in Cygnus X-3
• Relativistic Scattering in the Funnel of Cygnus X-3