Lepto-hadronic model for the broadband emission of Cygnus X-1

TL;DR

This paper develops a lepto-hadronic jet model to explain the broadband emission of Cygnus X-1, successfully fitting the observed spectrum and suggesting the MeV tail originates from jet synchrotron radiation.

Contribution

It introduces a comprehensive inhomogeneous jet model incorporating both leptonic and hadronic processes to interpret Cygnus X-1's spectral energy distribution.

Findings

The model fits the entire broadband spectrum, including the MeV tail.

Predicted hadronic gamma-ray emission is below current detection limits.

The radio emission is consistent with observations, though the model predicts a more compact jet.

Abstract

Cygnus X-1 is a well observed microquasar. Broadband observations at all wavelengths have been collected over the years. The origin of the MeV tail observed with COMPTEL and INTEGRAL is still under debate and it has mostly been attributed to the corona, although its high degree of polarization suggests it is synchrotron radiation from a jet. The origin of the transient emission above $\sim 100$ GeV is also unclear. We aim to disentangle the origin of the broadband spectral energy distribution (SED) of Cygnus X-1, focusing particularly on the gamma-ray emission, and to gain information on the physical conditions inside the jets. We develop and apply a lepto-hadronic, inhomogeneous jet model to the non-thermal SED of Cygnus X-1. We calculate the contributions to the SED of both protons and electrons accelerated in an extended region of the jet. We also estimate the radiation of charged secondaries produced in hadronic interactions, through several radiative processes. Absorption effects are considered. We produce synthetic maps of the jets at radio wavelengths. We find two sets of model parameters that lead to good fits of the SED. One of the models fits all the observations, including the MeV tail. This model also predicts hadronic gamma-ray emission slightly below the current upper limits. The flux predicted at 8.4 GHz is in agreement with the observations available in the literature, although the synthetic source is more compact than the imaged radio jet. Our results show that the MeV emission in Cygnus X-1 may be jet synchrotron radiation. This depends mainly on the strength of the jet magnetic field and the location of the injection region of the relativistic particles. Our calculations show that there must be energetic electrons in the jets quite far from the black hole.