One of the most important results on Galactic black holes that has come out of INTEGRAL was the discovery of the signature of strong polarization of the >300 keV spectrum of Cygnus X-1. This discovery is an indication that this spectral component is due to synchrotron emission, probably from the radio jet that is present in the hard state. In this two year, 2 Ms program we propose a continuation of these measurements with the aim of detecting changes in the hard state polarization properties, to set strong limits on the hard tail in the soft state, and to break degeneracies between the two families of models for the formation of the hard tail.
The INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched by the European Space Agency (ESA) on October 17, 2002, was designed to study high-energy phenomena in the universe. INTEGRAL was operating until february 2025 and it was equipped with three high-energy instruments: the Imager on Board the INTEGRAL Satellite (IBIS), the Spectrometer on INTEGRAL (SPI), and the JEM-X (Joint European Monitor for X-rays). Its Optical Monitoring Camera (OMC) provided optical V-band magnitude measurements, complementing the high-energy observations.
European Space Agency, Wilms, 2025, 'Continued Monitoring of the Polarization of Cygnus X-1', 1.0, European Space Agency, https://doi.org/10.57780/esa-2b12uaf
