Cygnus X-3 as a semi-hidden PeVatron

¹ Institutt for fysikk, NTNU, Trondheim, Norway
² Department of Physics, School of Natural Sciences, Technical University Munich, Garching, Germany

^⋆ Corresponding author

Received: 22 March 2025
Accepted: 18 June 2025

Abstract

Context. The high-mass X-ray binary Cygnus X-3 has long been suggested to be a source of high-energy photons and neutrinos.

Aims. In view of the increased sensitivity of current experiments, we examined the acceleration and interactions of high-energy cosmic rays (CRs) in this binary system, assuming that the compact object is a black hole.

Methods. Using a test-particle approach in a Monte Carlo framework, we employed magnetic reconnection or second-order Fermi acceleration and diffusive shock acceleration as the basic CR acceleration mechanisms.

Results. We found that in all three scenarios, CRs can be accelerated beyond PeV energies. High-energy photons and neutrinos are produced as secondaries in photo-hadronic interactions of CRs on X-ray photons and in the scattering on gas from the wind of the companion star. Normalising the predicted photon flux to the excess flux observed by LHAASO at energies above PeV in the direction of Cygnus X-3, a CR acceleration efficiency of 10⁻³ is sufficient to power the required CR luminosity. Our results suggest that the PeV photon flux from Cygnus X-3 could be in a bright phase that is significantly increased relative to the average flux of the past years.