Determine the origin of the high-energy gamma-ray tails of Cygnus X-1

Determine whether the approximately 0.1–10 MeV and 0.1–10 GeV gamma-ray tails of Cygnus X-1 in the hard state originate from the jet, the corona, hadronic processes, or a combination of these mechanisms.

Background

The appendix applies the coronal model developed for AGN to the hard-state spectrum of the stellar-mass black-hole binary Cygnus X-1. The observed spectrum contains two distinct high-energy tails, but their physical interpretation is unresolved. The paper identifies jet emission, coronal emission, and hadronic channels as competing or potentially complementary explanations, while noting that polarization measurements have been used to support a jet contribution. The authors then test whether a nonthermal coronal electron population can reproduce the broadband spectrum, but the broader question of the tails’ definitive origin remains open.

References

In its hard state, the $\gamma$-ray spectrum of Cygnus~X-1 exhibits two distinct high-energy tails, at $\sim 0.1$--$10$\,MeV and $0.1$--$10$\,GeV, whose origins remain debated~\citep[see][and references therein]{2025ApJ...993...54K}.

— Gamma-rays from black hole coronae: disentangling the leptonic and hadronic contributions  (2610.09676 - Bihan et al., 7 Oct 2026) in Appendix, Section A, “Application to Cygnus X-1”