# PoGO+ polarimetric constraint on the synchrotron jet emission of Cygnus X-1

## TL;DR

This study uses polarimetric data from PoGO+ to place constraints on the polarization and flux of potential synchrotron jet emission in Cygnus X-1's hard X-ray regime, finding it to be below certain upper limits.

## Contribution

It provides the first polarimetric constraints on the hard X-ray jet emission of Cygnus X-1, extending previous gamma-ray polarization studies into the hard X-ray band.

## Key findings

- Polarization fraction of the jet component is constrained to <10% for certain angles.
- Upper limit on polarized flux is <3 x 10^-9 erg s^-1 cm^-2.
- Jet emission polarization is below the detection threshold set by PoGO+.

## Abstract

We report a polarimetric constraint on the hard X-ray synchrotron jet emission from the Cygnus X-1 black-hole binary system. The observational data were obtained using the PoGO+ hard X-ray polarimeter in July 2016, when Cygnus X-1 was in the hard state. We have previously reported that emission from an extended corona with a low polarization fraction is dominating, and that the polarization angle is perpendicular to the disk surface. In the soft gamma-ray regime, a highly-polarized synchrotron jet is reported with INTEGRAL observations. To constrain the polarization fraction and flux of such a jet component in the hard X-ray regime, we now extend analyses through vector calculations in the Stokes QU plane, where the dominant corona emission and the jet component are considered simultaneously. The presence of another emission component with different polarization angle could partly cancel out the net polarization. The 90% upper limit of the polarization fraction for the additional synchrotron jet component is estimated as <10%, <5%, and <5% for polarization angle perpendicular to the disk surface, parallel to the surface, and aligned with the emission reported by INTEGRAL data, respectively. From the 20-180 keV total flux of 2.6 x 10^-8 erg s^-1 cm^-2, the upper limit of the polarized flux is estimated as <3 x 10^-9 erg s^-1 cm^-2.