Variability of X-ray polarization of Cyg X-1

Abstract

We present the results of a three-year X-ray, optical, and radio polarimetric monitoring campaign of the prototypical black hole X-ray binary Cyg X-1, conducted from 2022 to 2024. The X-ray polarization of Cyg X-1 was measured 13 times with the Imaging X-ray Polarimetry Explorer (IXPE), covering both hard and soft spectral states. The X-ray polarization degree (PD) in the hard state was found to be $\approx 4.0\%$, roughly twice as high as in the soft state, where it was around $2.2\%$. In both states, a statistically significant increase of PD with the energy was found. Moreover, a linear relation between PD and spectral hardness suggests a gradual and continuous evolution of the polarization properties, rather than an abrupt change of polarization production mechanism between states. The polarization angle (PA) was independent of the spectral state and showed no trend with the photon energy. The X-ray PA is well aligned with the orientation of the radio jet, as well as the optical and radio PAs. We find significant orbital changes of PA in the hard state, which we attribute to scattering of X-ray emission at intrabinary structure. No significant superorbital variability in PD or PA was found at the period $P_{\mathrm{so}}$ = 294 d. We also find no correlation between the X-ray and optical polarization; if any, there is a long-term anti-correlation between the X-ray PD and the radio PD.

12 pages, 10 figures, accepted for publication in Astronomy & Astrophysics

References in corpus (35)

• Modelling the behaviour of accretion flows in X-ray binaries
• CASA, the Common Astronomy Software Applications for Radio Astronomy
• WSClean: an implementation of a fast, generic wide-field imager for radio astronomy
• Cygnus X-1 contains a 21-solar mass black hole -- implications for massive star winds
• ixpeobssim: a Simulation and Analysis Framework for the Imaging X-ray Polarimetry Explorer
• Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1
• Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission
• Hot accretion flow in black hole binaries: a link connecting X-rays to the infrared
• Thermal disc emission from a rotating black hole: X-ray polarization signatures
• The synchrotron boiler and the spectral states of black hole binaries
• 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
• On the origin of spectral states in accreting black holes
• A transient relativistic radio jet from Cygnus X-1
• Polarized X-rays from windy accretion in Cygnus X-1
• Radio monitoring of the hard state jets in the 2011 outburst of MAXI J1836-194
• Polarization signatures of strong gravity in AGN accretion discs
• RoboPol: A four-channel optical imaging polarimeter
• RoboPol: AGN polarimetric monitoring data
• Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE
• A highly polarised radio jet during the 1998 outburst of the black hole transient XTE J1748-288
• Dramatic Drop in the X-Ray Polarization of Swift J1727.8$-$1613 in the Soft Spectral State
• Magnetic Field Properties inside the Jet of Mrk 421: Multiwavelength Polarimetry Including the Imaging X-ray Polarimetry Explorer
• Analytical techniques for polarimetric imaging of accretion flows in Schwarzschild metric
• Recovery of the X-ray polarisation of Swift J1727.8$-$1613 after the soft-to-hard spectral transition
• X-ray polarization from accretion disk winds
• X-ray polarimetry of the accreting pulsar GX 301-2
• Superorbital variability of X-ray and radio emission of Cyg X-1. I. Emission anisotropy of precessing sources
• X-ray Polarimetry of X-ray Pulsars
• Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry
• Comprehensive UV and Optical spectral analysis of Cygnus X-1: Stellar and wind parameters, abundances, and evolutionary implications
• Optical polarization signatures of black hole X-ray binaries
• Peering into the tilted heart of Cyg X-1 with high-precision optical polarimetry
• X-ray Polarization of the High-Synchrotron-Peak BL Lacertae Object 1ES 1959+650 during Intermediate and High X-ray Flux States
• Revealing two orthogonally polarized spectral components in Vela X-1 with IXPE
• Orbital variability of polarized X-ray radiation reflected from a companion star in X-ray binaries

Cited by in corpus (2)

• Assessing systematic uncertainties from spectral re-analysis of Cyg X-1 with different coronal geometries
• The constraining power of X-ray polarimetry: detailed structure of the intrabinary bow shock in Cygnus X-3