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Microquasars as the major contributors to Galactic cosmic rays around the "knee"

Samy Kaci, Gwenael Giacinti, Felix Aharonian, Jie-Shuang Wang

Abstract

Recently, LHAASO detected a gamma-ray emission extending beyond $100\,\rm{TeV}$ from 4 sources associated to powerful microquasars. We propose that such sources are the main Galactic PeVatrons and investigate their contribution to the proton and gamma-ray fluxes by modeling their entire population. We find that the presence of only $\sim10$ active powerful microquasars in the Galaxy at any given time is sufficient to account for the proton flux around the knee and to provide a very good explanation of cosmic-ray and gamma-ray data in a self-consistent picture. The $10\,\rm{TeV}$ bump and the $300\,\rm{TeV}$ hardening in the cosmic-ray spectrum naturally appear, and the diffuse background measured by LHAASO above a few tens of $\rm{TeV}$ is accounted for. This supports the paradigm in which cosmic rays around the knee are predominantly accelerated in a very limited number of powerful microquasars.

Microquasars as the major contributors to Galactic cosmic rays around the "knee"

Abstract

Recently, LHAASO detected a gamma-ray emission extending beyond from 4 sources associated to powerful microquasars. We propose that such sources are the main Galactic PeVatrons and investigate their contribution to the proton and gamma-ray fluxes by modeling their entire population. We find that the presence of only active powerful microquasars in the Galaxy at any given time is sufficient to account for the proton flux around the knee and to provide a very good explanation of cosmic-ray and gamma-ray data in a self-consistent picture. The bump and the hardening in the cosmic-ray spectrum naturally appear, and the diffuse background measured by LHAASO above a few tens of is accounted for. This supports the paradigm in which cosmic rays around the knee are predominantly accelerated in a very limited number of powerful microquasars.

Paper Structure

This paper contains 1 equation, 4 figures.

Figures (4)

  • Figure 1: Cosmic-ray proton flux at $3.5\,\rm{PeV}$ (normalized to the Icetop and LHAASO measurements icetoplhaaso_proton) over a circle of radius $8.5\,\rm{kpc}$ centered around the Galactic center for different realizations of the Galaxy. The angle $\theta$ represents different positions on the circle.
  • Figure 2: Contribution of microquasars to the proton spectrum. In both panels, the dashed cyan curve shows the flux from a standard population of Galactic sources producing a power-law spectrum with an exponential cutoff at $260\,\rm{TeV}$, the magenta shaded area shows the flux contributed by microquasars assumed to produce a power-law spectrum of index $2$ with an exponential cutoff at $10\,\rm{PeV}$ and the purple curve represents the sum of the cyan and the magenta curves. The upper panel shows the sole contribution of the sea of cosmic rays due to miroquasars and the lower panel shows the total contribution of microquasars. The data points are from DAMPE dampe, CREAM cream, Icetop icetop and LHAASO lhaaso_proton.
  • Figure 3: Histogram displaying the number of microquasars detected in the FOV of LHAASO at $100\,\rm{TeV}$ per simulation over $100$ simulations. The blue histogram represents the number of detections without MCs and the yellow dashed histogram represents the number of detections with molecular clouds with a density $n=30\,\rm{cm}^{-3}$ and a radius $r=50\,\rm{pc}$. The blue (red) vertical dashed line represents the average number of detections without (with) MCs.
  • Figure 4: Contribution of microquasars to the Galactic diffuse gamma-ray emission reported by LHAASO lhaaso_diffuse. The upper (lower) panel shows the data related to the inner (outer) Galaxy defined as the region $15^{\circ}\leq l<125^{\circ}$ and $\left|b\right|\leq5^{\circ}$ ($125^{\circ}\leq l<235^{\circ}$ and $\left|b\right|\leq5^{\circ}$). The contribution of unresolved pulsars shown in green is taken from me2. In both panels, the yellow area represents the sum of the contributions of microquasars (magenta area), pulsars (green area) and SNRs (dashed cyan curve) assumed to have a cutoff at $200\,\rm{TeV}$. The shaded areas represent one standard deviation over $100$ simulations.