Microquasar Remnants as Pevatrons Illuminating the Galactic Cosmic Ray Knee

Abstract

Microquasars are primary candidates for Galactic PeVatrons, yet their collective contribution to the cosmic ray (CR) ``knee" remains poorly understood. We investigate this contribution by simulating anisotropic diffusive propagation through the Galactic magnetic field (GMF). Our results demonstrate that the GMF establishes a transport regime where magnetic connectivity between sources and the solar neighborhood determines the local flux. Active sources aligned with local GMF lines, such as Cygnus X-1, exhibit significant flux enhancements, while magnetically disconnected sources, such as V616 Mon, are strongly suppressed. By integrating source evolution with anisotropic transport, we show that the observed proton bump at the CR ``knee" is best reproduced by the cumulative contribution of microquasar remnants, which is often dominated by a few nearby or recent events, rather than the active ones alone. We find that a harder injection spectrum allows CRs from remnants to reproduce the PeV bump after propagation, as low-energy CRs have sufficient time to accumulate while high-energy CRs escape the Galactic plane. Our findings suggest that the integrated history of microquasar remnants, governed by the interplay of source age and magnetic connectivity, is the primary driver populating the observed CR ``knee''.

Figures (4)

• Figure 1: Illustration of the spatial distribution of 10 PeV cosmic rays (colored dots) from known Galactic microquasar locations (stars). The CR transport is influenced by the filamentary morphology of the Galactic magnetic field (gray spirals). For this comparison, we assume a uniform source age and jet duration of $\tau_{\rm age} = \tau_{\rm dur} = 0.2$ Myr with an anisotropy level of $\varepsilon = 0.1$.
• Figure 2: Predicted CR spectra from Cyg X-1 and V616 Mon locations compared with LHAASO LHAASO:2025byy and IceTop IceCube:2019hmk data. Results show anisotropic diffusion ($\varepsilon = 0.1, 0.01$) at $\tau_{\rm age} = 0.2$ Myr (top) and $\tau_{\rm age} = 1$ Myr (bottom). We assume Model A injection with $M_{\rm BH} = 10 M_\odot$, $\tau_{\rm dur} = 0.2$ Myr, and $\eta_{\rm cr} = 0.005$.
• Figure 3: Diffuse CR energy spectra for Monte Carlo ensembles (shaded bands) of microquasars, assuming an anisotropic diffusion coefficient of $\varepsilon = 0.1$. Each ensemble represents a realization of the Galactic microquasar remnants population. The spectrum from injection Model A (red) with $\eta_{\rm cr} = 0.005$ is compared against a power-law scenario (orange) characterized by $s_{\rm cr} = 2$, $E_{\rm max} = 6$ PeV, and $\eta_{\rm cr} = 0.03$.
• Figure 4: Cosmic ray energy density at 1 PeV shown as a function of distance and source age. The model assumes a jet duration $\tau_{\rm dur} = 0.2$ Myr from a $10 M_\odot$ black hole and isotropic diffusion without GMF effects. The injection spectrum $Q(E)$ is normalized such that the total cosmic ray luminosity is $L_{\rm cr} = \eta_{\rm cr} L_{\rm Edd}$, where we adopt $\eta_{\rm cr} = 0.1$. The spectral index is assumed to be $s_{\rm cr} = 2$. Contours identify density levels between $4 \times 10^{-7}$ and $4 \times 10^{-5}$ eV cm$^{-3}$.