Polarized X-rays from windy accretion in Cygnus X-1

Abstract

Recent X-ray polarimetric data on the prototypical black hole X-ray binary Cyg~X-1 from the Imaging X-ray Polarimetry Explorer present tight constraints on accretion geometry in the hard spectral state. Contrary to general expectations of a low, <1% polarization degree (PD), the observed average PD was found to be a factor of 4 higher. Aligned with the jet position angle on the sky, the observed polarization favors geometry of the X-ray emission region stretched normally to the jet in the accretion disk plane. The high PD is, however, difficult to reconcile with the low orbital inclination of the binary $i\approx 30$ deg. We suggest that this puzzle can be explained if the emitting plasma is outflowing with a mildly relativistic velocity $\gtrsim 0.4c$. Our radiative transfer simulations show that Comptonization in the outflowing medium elongated in the plane of the disk radiates X-rays with the degree and direction of polarization consistent with observations at $i\approx 30$ deg.

11 pages, 6 figures, ApJL, in press

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• Modelling the behaviour of accretion flows in X-ray binaries
• Cygnus X-1 contains a 21-solar mass black hole -- implications for massive star winds
• 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
• On the origin of spectral states in accreting black holes
• Evolution of the 0.01 - 25 Hz power spectral components in Cygnus X-1

Cited by in corpus (2)

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• A quantitative explanation of the radio--X-ray correlation in black-hole X-ray binaries