Peering into the tilted heart of Cyg X-1 with high-precision optical polarimetry

Abstract

We present the high-precision optical polarimetric observations of black hole X-ray binary Cyg X-1, spanning several cycles of its 5.6 day orbital period. Week-long observations on two telescopes located in opposite hemispheres allowed us to track the evolution of the polarization within one orbital cycle with the highest temporal resolution to date. Using the field stars, we determine the interstellar polarization in the source direction and subsequently its intrinsic polarization. The optical polarization angle is aligned with that in the X-rays as recently obtained with the Imaging X-ray Polarimetry Explorer. Furthermore, it is consistent, within the uncertainties, with the position angle of the radio ejections. We show that the intrinsic PD is variable with the orbital period with the amplitude of $\sim$0.2% and discuss various sites of its production. Assuming the polarization arises from a single Thomson scattering of the primary star radiation by the matter that follows the black hole in its orbital motion, we constrain the inclination of the binary orbit $i>120^{\circ }$ and its eccentricity $e<0.08$. The asymmetric shape of the orbital profiles of Stokes parameters implies also the asymmetry of the scattering matter distribution about the orbital plane, which may arise from the tilted accretion disk. We compare our data to the polarimetric observations made over 1975-1987 and find good, within $1^{\circ }$, agreement between the intrinsic polarization angles. On the other hand, the PD decreased by 0.4% over half a century, suggesting the presence of secular changes in the geometry of accreting matter.

12 pages, 18 figures

References in corpus (11)

• Cygnus X-1 contains a 21-solar mass black hole -- implications for massive star winds
• Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1
• Superorbital variability of X-ray and radio emission of Cyg X-1 - II. Dependence of the orbital modulation and spectral hardness on the superorbital phase
• A transient relativistic radio jet from Cygnus X-1
• Black hole spin-orbit misalignment in the X-ray binary MAXI J1820+070
• Multi-Wavelength Variability. Accretion and Ejection at the Fastest Timescales
• Orbital variability of the optical linear polarization of the $\gamma$-ray binary LS I +61 303 and new constraints on the orbital parameters
• Superorbital variability of X-ray and radio emission of Cyg X-1. I. Emission anisotropy of precessing sources
• Disc and wind in black hole X-ray binary MAXIJ1820+070 observed through polarized light during its 2018 outburst
• Evidence for a black hole spin--orbit misalignment in the X-ray binary Cyg X-1
• Optical polarization signatures of black hole X-ray binaries

Cited by in corpus (5)

• Variability of X-ray polarization of Cyg X-1
• XRISM High-Resolution X-ray Spectroscopy of Cygnus X-1 -- highly ionized Iron absorption structures
• Orbital variability of polarized X-ray radiation reflected from a companion star in X-ray binaries
• Optical polarimetry of the accreting black hole X-ray binary Swift J1727.8$-$1613 over the state transition and radio ejections
• Unprecedentedly bright X-ray flaring in Cygnus X-1 observed by INTEGRAL