Very-high-energy gamma rays from cosmic rays escaping from Galactic black hole binaries

TL;DR

This paper models cosmic-ray diffusion from Galactic black hole binaries to explain observed very-high-energy gamma rays, revealing a common spectral pattern and supporting their role as sources of Galactic cosmic rays beyond PeV energies.

Contribution

It provides a unified diffusion model that explains extended gamma-ray emission from multiple microquasars and suggests they share similar cosmic-ray spectra and diffusion properties.

Findings

Gamma-ray spectra are described by broken power laws with spectral breaks.

All five microquasars have the same cosmic-ray energy spectrum, $dN/dE \,\propto\, E^{-2}$.

The model supports black hole binaries as sources of ultra-high-energy Galactic cosmic rays.

Abstract

We solve the cosmic-ray diffusion around a Galactic black hole binary (microquasars) by considering the finite size of the escape region and the continuous cosmic-ray injection. We find that the energy spectrum of escaping cosmic rays in the gamma-ray emission region is described by a broken power law spectrum with one or two spectral breaks even though the total spectrum of escaping cosmic rays is a single power law spectrum. Using the solution for the diffusion equation, we construct a unified picture that explains spatially extended very-high-energy gamma rays from five microquasars observed by HAWC and LHAASO. The comparison of our unified model and observed data suggest that all five microquasars have the same energy spectrum of the escaping CRs, $dN/dE \propto E^{-2}$, the same diffusion coefficient, and the same emission region. The hard energy spectrum without the high-energy cutoff supports the idea that the origin of Galactic CRs beyond PeV energies is Galactic black hole binaries.