2015ApJ...807...17R

Polarization of the keV hard tail of the microquasar Cygnus X-1 has been independently reported by INTEGRAL/Imager on Board the INTEGRAL Satellite (IBIS), and INTEGRAL/SPectrometer on INTEGRAL and interpreted as emission from a compact jet. These conclusions were, however, based on the accumulation of all INTEGRAL data regardless of the spectral state. We utilize additional INTEGRAL exposure accumulated until 2012 December, and include the AMI/Ryle (15 GHz) radio data in our study. We separate the observations into hard, soft, and intermediate/transitional states and detect radio emission from a compact jet in hard and intermediate states (IS), but not in the soft. The 10-400 keV INTEGRAL (JEM-X and IBIS) state resolved spectra are well modeled with thermal Comptonization and reflection components. We detect a hard tail in the 0.4-2 MeV range for the hard state only. We extract the state dependent polarigrams of Cyg X-1, which are all compatible with no or an undetectable level of polarization except in the 400-2000 keV range in the hard state where the polarization fraction is 75%±32% and the polarization angle 40.°0±14.°3. An upper limit on the 0.4-2 MeV soft state polarization fraction is 70%. Due to the short exposure, we obtain no meaningful constraint for the IS. The likely detection of a keV polarized tail in the hard state, together with the simultaneous presence of a radio jet, reinforce the notion of a compact jet origin of the keV emission.