Investigating the origin of the far-infrared emission of the microquasar Cygnus X-1

Microquasars are Galactic Xray binaries exhibiting collimated outflows commonly called jets. In particular, the socalled compact jets are detected almost simultaneously in all the spectral domains, and are characterised by the existence of a spectral break at which they change their emission regime from optically thick to optically thin synchrotron. The determination of this cutoff frequency is fundamental for the understanding of the accretionejection processes as it is related to the black hole spin and mass, as well as to the accretion rate. In a previous Spitzer spectroscopic study of Cygnus X1, we assessed the contribution of the compact jets to the midinfrared continuum as well as their spectral break. Nevertheless, its accurate value appears to be dependent on the model used to describe the continuum of the companion star, which is the blue supergiant HD226868. Indeed, it changes whether we consider bremsstrahlung from the stellar winds or thermal emission from a circumstellar dust component. We therefore require photometric observations of Cygnus X1 with PACS, in the blue, green, and red filters, in order to assess the flux density level of the source at 70, 100, and 160 microns. These measurements, combined with our Spitzer spectra, will allow us to discriminate between bremsstrahlung and dust, which will eventually lead us to the accurate determination of the spectral break of the compact jets exhibited by Cygnus X1. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]