High energy gamma rays from nebulae associated with extragalactic microquasars and ultra-luminous X-ray sources

In the extragalactic sky, microquasars and ultra-luminous X-ray sources (ULXs) are known as energetic compact objects locating at off-nucleus positions in galaxies. Some of these objects are associated with expanding bubbles with a velocity of 80-250 kms−1. We investigate the shock acceleration of particles in those expanding nebulae. The nebulae having fast expansion velocity ≳120kms−1 are able to accelerate cosmic rays up to ∼100 TeV. If 10% of the shock kinetic energy goes into particle acceleration, powerful nebulae such as the microquasar S26 in NGC 7793 would emit gamma rays up to several tens TeV with a photon index of ∼2. These nebulae will be good targets for future Cherenkov Telescope Array observations given its sensitivity and angular resolution. They would also contribute to ∼7% of the unresolved cosmic gamma-ray background radiation at ≥ 0.1 GeV. In contrast, particle acceleration in slowly expanding nebulae ≲120kms−1 would be less efficient due to ion-neutral collisions and result in softer spectra at ≳ 10 GeV.