Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1

TL;DR

This study measures gamma-ray polarization from Cygnus X-1, revealing two emission mechanisms with different polarization properties, providing insights into the source's emission processes and jet activity.

Contribution

First polarization measurement of gamma-ray emission from Cygnus X-1, distinguishing emission mechanisms and linking MeV emission to jet activity.

Findings

250-400 keV emission is weakly polarized and dominated by thermal Compton scattering

400 keV-2 MeV emission is strongly polarized, likely related to the jet

Results suggest different origins for the two spectral components

Abstract

Because of their inherently high flux allowing the detection of clear signals, black hole X-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the INTEGRAL/IBIS telescope. Spectral modeling of the data reveals two emission mechanisms: The 250-400 keV data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400keV-2MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band.