Polarized X-Rays from Windy Accretion in Cygnus X-1



: Poutanen Juri, Veledina Alexandra, Beloborodov Andrei M

Publisher: IOP Publishing Ltd

: 2023

: Astrophysical Journal Letters

: ASTROPHYSICAL JOURNAL LETTERS

: ASTROPHYS J LETT

:  L10

: 949

: 1

: 9

: 2041-8205

DOI: https://doi.org/10.3847/2041-8213/acd33e

: https://iopscience.iop.org/article/10.3847/2041-8213/acd33e

: https://research.utu.fi/converis/portal/detail/Publication/179723302


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, less than or similar to 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 approximate to 30 degrees. We suggest that this puzzle can be explained if the emitting plasma is outflowing with a mildly relativistic velocity greater than or similar to 0.4 c. Our radiative transfer simulations show that Comptonization in the outflowing medium elongated in the plane of the disk and radiates X-rays with the degree and direction of polarization consistent with observations at i approximate to 30 degrees.

Last updated on 2025-20-02 at 10:18