Spectrum of Relativistic and Subrelativistic Cosmic Rays in the 100 pc Central Region

TL;DR

This study derives the spectrum and propagation characteristics of relativistic and subrelativistic cosmic rays near the Galactic Center, revealing their significant energy density and potential contributions to observed X-ray and gamma-ray emissions.

Contribution

It provides a detailed analysis of cosmic ray spectra and propagation modes in the Sgr B2 cloud, highlighting the role of magnetic scattering and the energy distribution of subrelativistic protons.

Findings

Subrelativistic protons have a hard spectrum with spectral index > -1.

Energy density of subrelativistic protons exceeds that of relativistic CRs by an order of magnitude.

Subrelativistic protons contribute to 6.4 keV emission and may be detectable via high-energy X-ray excess.

Abstract

From the rate of hydrogen ionization and the gamma ray flux, we derived the spectrum of relativistic and subrelativistic cosmic rays (CRs) nearby and inside the molecular cloud Sgr B2 near the Galactic Center (GC). We studied two cases of CR propagation in molecular clouds: free propagation and scattering of particles by magnetic fluctuations excited by the neutral gas turbulence. We showed that in the latter case CR propagation inside the cloud can be described as diffusion with the coefficient $\sim 3\times 10^{27}$ cm$^2$ s$^{-1}$. For the case of hydrogen ionization by subrelativistic protons, we showed that their spectrum outside the cloud is quite hard with the spectral index $\delta>-1$. The energy density of subrelativistic protons ($>50$ eV cm$^{-3}$) is one order of magnitude higher than that of relativistic CRs. These protons generate the 6.4 keV emission from Sgr B2, which was about 30\% of the flux observed by Suzaku in 2013. Future observations for the period after 2013 may discover the background flux generated by subrelativistic CRs in Sgr B2. Alternatively hydrogen ionization of the molecular gas in Sgr B2 may be caused by high energy electrons. We showed that the spectrum of electron bremsstrahlung is harder than the observed continuum from Sgr B2, and in principle this X-ray component provided by electrons could be seen from the INTEGRAL data as a stationary high energy excess above the observed spectrum $E_x^{-2}$.