Refereed journal article or data article (A1)
Polarized X-Rays from Windy Accretion in Cygnus X-1
List of Authors: Poutanen Juri, Veledina Alexandra, Beloborodov Andrei M
Publisher: IOP Publishing Ltd
Publication year: 2023
Journal: Astrophysical Journal Letters
Journal name in source: ASTROPHYSICAL JOURNAL LETTERS
Journal acronym: ASTROPHYS J LETT
Article number: L10
Volume number: 949
Issue number: 1
Number of pages: 9
ISSN: 2041-8205
DOI: http://dx.doi.org/10.3847/2041-8213/acd33e
URL: https://iopscience.iop.org/article/10.3847/2041-8213/acd33e
Self-archived copy’s web address: 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.
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