A Multi-messenger Search for a Nearby Microquasar Contributor to the Cosmic Ray Knee

Recently, LHAASO has detected five microquasars with high confidence, which are associated with SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1, respectively. Except for Cygnus X-1, the maximum energies of gamma-ray photons emitted from these sources all exceed 100 TeV, strongly suggesting that microquasars are capable of accelerating cosmic-ray particles to energies above the PeV range. This work investigates the origin of the cosmic-ray knee region based on gamma-ray observational data from the aforementioned sources, combined with cosmic-ray proton, helium, and all-particle energy spectra, as well as anisotropy observations. Calculations indicate that these known sources contribute negligibly to the cosmic-ray knee region. However, further joint analysis reveals that a single microquasar located in a region approximately on the 2.6 kiloparsec scale in the anti-Galactic center direction can reasonably reproduce the observed cosmic-ray proton, helium, and all-particle energy spectra, as well as anisotropy features detected near Earth. We propose that this region may host one or several unidentified microquasars or similar systems, whose accelerated cosmic rays could dominate the observational characteristics of the knee region.

💡 Research Summary

The paper investigates whether the recently discovered ultra‑high‑energy γ‑ray emitting microquasars can account for the “knee” feature in the Galactic cosmic‑ray spectrum around a few peta‑electronvolts (PeV). LHAASO has identified five microquasars with high confidence—SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X‑1—four of which emit photons above 100 TeV, suggesting they accelerate particles to PeV energies. The authors first construct a conventional background model in which supernova remnants (SNRs) dominate cosmic rays below the knee. Using a diffusion equation with a rigidity‑dependent diffusion coefficient D(R)=D₀ β (R/R₀)^δ (δ≈0.33, D₀≈2×10²⁸ cm² s⁻¹) and a broken‑power‑law source spectrum (break at ~200 GV, indices ν₁≈2.4, ν₂≈2.8, exponential cutoff at ~200 TV), they reproduce the measured B/C ratio and the low‑energy proton, helium and all‑particle spectra from AMS‑02, DAMPE, Voyager, etc.

Next, the γ‑ray data from LHAASO are used to constrain the individual particle injection spectra of the four microquasars (excluding Cygnus X‑1). By fitting the observed γ‑ray fluxes with a hadronic model, the authors infer each source’s total proton luminosity (L₀∼10³⁸–10³⁹ erg s⁻¹) and spectral index β≈2.2–2.5. When propagated through the same diffusion halo as the SNR background, the contributions of these known microquasars to the Earth‑observed cosmic‑ray flux are found to be ≤1 % of the total, i.e., negligible for the knee region.

The central hypothesis is that the knee is dominated by a single nearby accelerator, plausibly an as‑yet unidentified microquasar or a similar binary system, located roughly opposite the Galactic centre (anti‑GC direction). The authors model this source as an instantaneous injection with spectrum q(R)=q₀ (R/R₀)^{-β} exp(−R/R_cut,loc). The propagated flux at Earth is given by a Gaussian diffusion solution with σ(R,t)=√