# High Energy Gamma Rays from Nebulae Associated with Extragalactic Microquasars and Ultra-Luminous X-ray Sources

## TL;DR

This paper explores how nebulae associated with extragalactic microquasars and ULXs can accelerate particles to high energies, producing gamma rays detectable by future telescopes and contributing to cosmic gamma-ray background.

## Contribution

It demonstrates that fast-expanding nebulae can accelerate cosmic rays up to 100 TeV and predicts gamma-ray emissions, highlighting their potential as targets for upcoming observations.

## Key findings

- Nebulae with expansion velocities >120 km/s can accelerate particles up to 100 TeV.
- Powerful nebulae may emit gamma rays up to tens of TeV with a photon index of ~2.
- Nebulae could contribute ~7% to the unresolved cosmic gamma-ray background.

## Abstract

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 ${\rm km~s^{-1}}$. We investigate the shock acceleration of particles in those expanding nebulae. The nebulae having fast expansion velocity $\gtrsim120~{\rm km~s^{-1}}$ are able to accelerate cosmic rays up to $\sim100$ 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 $\sim2$. These nebulae will be good targets for future Cherenkov Telescope Array observations given its sensitivity and angular resolution. They would also contribute to $\sim7$% of the unresolved cosmic gamma-ray background radiation at $\ge0.1~{\rm GeV}$. In contrast, particle acceleration in slowly expanding nebulae $\lesssim120~{\rm km~s^{-1}}$ would be less efficient due to ion-neutral collisions and result in softer spectra at $\gtrsim10$ GeV.