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

Abstract

Context. 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 that it is synchrotron radiation from a jet. The origin of the transient emission above ∼100 GeV is also unclear. Aims. We aim to clarify 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. Methods. We developed a lepto-hadronic, inhomogeneous jet model and applied it to the non-thermal SED of Cygnus X-1. We calculated the contributions to the SED of both protons and electrons accelerated in an extended region of the jet. We also estimated the radiation of charged secondaries produced in hadronic interactions through several radiative processes. Absorption effects were considered. We produced synthetic maps of the jets at radio wavelengths. Results. 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. Conclusions. 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.

Gamma rays detected from Cygnus X-1 with likely jet origin

“…Under the assumption of Pass 8 sensitivity, 10 yr of Fermi-LAT data will allow to constrain an eventual cutoff below 100 GeV, whereas CTA will be more fsensitive above this energy in 200 hr of observation. CTA could still detect a different emission component, possibly of hadronic origin (Pepe et al 2015). The sensitivity curve of CTA for 200 hr of observations with the North array is included in the broad-band SED shown in Figure 7.…”

Extending the ‘energetic scaling of relativistic jets from black hole systems’ to include γ-ray-loud X-ray binaries

“…Lepto-hadronic models for the broadband emission of Cyg X-1 by Pepe et al (2015) also favours a synchrotron or SSC, and therefore jet origin, for the high energy tail. The low mass of the companion to V404 Cyg, the temporal association of γ-ray excess and radio flares, and the simultaneous detection of the 511 keV annihilation line and γ-rays of higher energy all point to the jet as the origin for such emission (Loh et al 2016).…”

“…Values for the inclination are listed in Table 1. XRB jets typically have Γ < 5. For Cyg X-1 a Lorentz factor Γ = 1.25 is used by Pepe et al (2015) for modelling the lepto-hadronic broadband emission, whilst from the jet opening angle and β cos i from MFN06, there is a minimum value of Γ = 3.3. We show results for both values.…”

Modeling the polarization of high-energy radiation from accreting black holes

“…For models with more realistic particle distributions, the synchrotron fluxes at E ∼ MeV are lower than those observed in Cygnus X-1. Pepe et al (2015) recently obtained good fits to the MeV emission for several sets of parameters. Nevertheless, current models for the polarization of the jet have problems to reproduce the polarization angle measured in Cygnus X-1.…”