Origin of Multi-band Emission from the Microquasar Cygnus X--1

Abstract

We study the origin of non-thermal emissions from the Galactic black hole X-ray binary Cygnus X--1 which is a confirmed high mass microquasar. By analogy with those methods used in studies of active galactic nuclei, we propose a two-dimensional, time-dependent radiation model from the microquasar Cygnus X--1. In this model, the evolution equation for relativistic electrons in a conical jet are numerically solved by including escape, adiabatic and various radiative losses. The radiative processes involved are synchrotron emission, its self-Compton scattering, and inverse Compton scatterings of an accretion disk and its surrounding stellar companion. This model also includes an electromagnetic cascade process of an anisotropic $\gamma$-$\gamma$ interaction. We study the spectral properties of electron evolution and its emission spectral characteristic at different heights of the emission region located in the jet. We find that radio data from Cygnus X--1 are reproduced by the synchrotron emission, the \emph{Fermi} LAT measurements by the synchrotron emission and Comptonization of photons of the stellar companion, the TeV band emission fluxes by the Comptonization of the stellar photons. Our results show that: (1) Radio emission region extends from the binary system scales to the termination of the jet. (2) The GeV band emissions should originate from the distance close to the binary system scales. (3) The TeV band emissions could be inside the binary system, and these emissions could be probed by the upcoming CTA telescope. (4) The MeV tail emissions, which produces a strongly linearly polarized signal, are emitted inside the binary system. The location of the emissions is very close to the inner region of the jet.